Hi there. The Alaska is a good choice
I don't think the Billy is big enough for a Syrian - it's not very deep. What is your budget? There is also the Barney and Alexander cages on Zooplus which are good for Syrians, but it depends if you have space for a 100cm cage. I think the Barney is a good buy. It's only about £10 to £15 more than the Alaska, but comes with a good house for a Syrian (which the Alaska doesn't so you'd need a house) and a good shelf. It also comes with a basic 8" wheel, which would do for a start, but an 11" wheel would be better.
Hamster Cages | great selection at zooplus
I have the Barney for our Syrian
The Alaska is fine too. Some people find they want to upgrade from it after a while though, but it's a perfectly adequate size.