It's difficult isn't it . I thought along the same lines as you when I got Charlie about 6 weeks ago. I got a Rotastack Space Command which had a big (I thought) base unit and three other smaller units and lots of tubes connecting them. In a way I am glad that Charlie grew too big for the tubes (very quickly!) because it made me read up about Hamster housing and realise his current set up was too restrictive - and I had already noticed he wasn't moving around that much and sleeping a lot.
I think what you might be able to do is buy a cage that attaches to the crittertrail units you have now so he can go into a bigger space as well. But then that would spread out a bit space wise, and if you get a cage you might as well get rid of the Crittertrail! I was totally p'd off after spending nearly £40 to find I needed to get something else. It is good that your syrian is quite small now fully grown with regard to the tubing. I have just looked up Crittertrail units and there are quite a few different sizes - some are big, some are small - which ones do you have please? That might make it easier to see if she needs more space or not, but really, as Nice Crocs says, it is the big floor area they need and the biggest crittertrail I saw was 25cm by 47 cm, even though it is tall. Could you add a photo?
The RSPCA say minimum floor area should be 75cm by 50cm - you might have that if you double up the floor area of the two cages, but as Nice Crocs says, it really needs to be one area of floor space, not divided into separate rooms. Unfortunately, manufacturers are just out to make profits and even some bigger cages come with wheels that are too small etc. In a big cage of 75 x 50 you can put little nest areas so they can still feel cosy but have plenty of space to explore, bars to climb etc - it is like a small house in a big habitat (ie a world to explore!), instead of a big house with lots of rooms, but no habitat. Charlie didn't like being moved - it took him two weeks to settle into his new big cage, but now he climbs bars, goes foraging, hides behind things and loves his little pod house and interacts with us more. I am wishing I had bought an even bigger cage! If you don't want to get into diy (I didn't - I hate it and am not very good at it) you won't want one with wood shelves etc that need waterproofing, or a bin cage (which could be a cheap option if you are ok with diy).
If I were you I would get the Barney cage from zooplus but don't put the wooden shelf in. It is less than £40 and a good cage. It comes with a nice house and then you just need a wheel and a few toys - a corner potty litter tray is good too, because they tend to use them and then the cage doesn't get smelly. It is 82cm x 52cm maximum size (although the base is slightly smaller), or the Hamster Heaven might be good for you - it still has tubes if your hamster likes them and isn't too big, although personally I would never have tubes again! It comes with everything - little pod house, platforms etc, although I think the wheel is too small and would need replacing, so ends up not being cheaper than the other cage and buying things separately, and it is supposed to be a big of a pig to put together (and therefore take apart for cleaning). It is about £60. But from what I have heard, Hamsters tend to use the top penthouse to sleep in and end up weeing in it too and it is messy. Plus I am not convinced about the corner tubes (Charlie got stuck in one of those when we had it inside his new cage - they are too flat and the angle is too much - and your Hamster only has to be feeling unwell one day). Personally I would never have tubes again! And there are stories of Hamsters pushing the tubes out or chewing them out and escaping. I reckon cost wise, the Barney cage is a good buy - just leave the platform out, keep the house then all you need is a wheel, food dish and water bottle (I always have two water bottles in case one fails), a plastic corner tray for potty litter and a few toys - you could fit some really nice big toys in a cage like that which means you won't need a platform- see below. She will have bars to climb, lots of space to explore and run around in. I've added some links with photos below. I thought I didn't have enough space for a big cage, but it makes you get creative! I moved the room around, put a load of stuff on shelves and had a good tidy and made a lovely big space against a wall for the cage. If the Barney is TOO big, there are others, but I think it is the cheapest.
If I was starting from scratch again, and learning what I have so far, I would get the barney cage - this ladder with food bowl (below), and a big 29cm floor standing wheel - then all you need is water bottle, a chew stick or log and you could put a dish of some kind in with chinchilla bathing sand and watch her have a sand bath and also have room for a climbing toy - you can add things a bit at a time and make home made things. Some people find they like cardboard toys best and they are safe - and old shoe box with a hole cut in and a cardboard tube from the centre of a kitchen roll. Charlie loves his cardboard tube! And loads of space to roam, burrow and roll in.
You could keep one of your Crittertrails as a carry case in case you ever need to take her to the vets or anywhere, or as a temporary playground while you are cage cleaning, so could put it away in a cupboard somewhere.
Run your mouse over the first photo and they will open up bigger so you can scroll through.
Have a look at the reviews for the Barney cage
Great Deals on Cages at zooplus!: Barney Pet Cage
but I would definitely leave the wood platform and ladder out or they will get stinky. The house is ok as it is hollow underneath.
http://www.amazon.co.uk/Kerbl-Nature...hamster+ladder
this is really big and will go most of the length of the cage - it has ladders to climb and an inset food bowl. The knob comes off if it is in the way.
Hamster Potty corner Litter Tray for Mice Hamster Gerbil Cage House. Size: 16 × 7 × 12/12 cm: Amazon.co.uk: Pet Supplies
This is the corner potty tray I use - you put potty litter in it and it stays fresh, hamster wants to wee in it as it is in a corner, and it only needs emptying and new litter in about every week to 10 days.
http://www.amazon.co.uk/Rosewood-Bor...ewood+log+roll
This is a chew and a toy, lasts for ages - Charlie likes to nibble at it and climb over it.
And that's about all you need, apart from a cardboard tube, a water bottle and the shavings or bedding in the bottom, plus some white paper so she can built a nest in the house. You already have a 12" silent spinner.
Photos of these products - and have a look around on here at other peoples' set ups, but I'm not that clever with making habitats and like to keep it simple!: