Sherlines are sort of puny, they will never be as rigid as any of the sieg models (the harbor freight model is a sieg model)
Also, the HF 7x10 is not 7x10, its more 7x8. The 7x12 is 4 inches longer than the 7x10 so it is actually 7x12.
I would recommend looking around a bit more online, check out mini-lathe.com. I think there is a lot of info there about the 7x series. There are a few yahoo groups for the other lathes as well.
I would do a lot of research before you pick one up, you really don't want to get stuck with something that's not right for you.
Grizzly has some good lathes. I just bought the 10 x 22 inch metal lathe. it cost 1250 after freight shipping and some tooling bits. Jet is also decent brand.