If you had the choice what would you buy a set of rollers or a Turbo Trainer...
if i wanted something a bit more like riding on the road (balance, handling) - rollers.
if i wanted something a bit more straightforward, get on, ride (and do that interval/sufferfest/etc) then i'd probably get a turbo. little bit smaller, too.
i've got a turbo and i hate it. i'd rather ride in the freezing rain and pitch black of mid-winter (and do). i use it for structured sessions, and only when i'm feeling particularly at peace with life that making myself sweat like a pig in my garage seems like a good idea.
don't let my personal issues put you off though - they are great at what they do and there are loads of people that swear by them. i swear at them, instead.
Turbo trainer. I think it improves your cardiovascular system more.
Rollers for me - slow down and you fall off so it's a good workout. I bought Cycleops rollers thinking I would get the optional resistance unit later, but there's no need, I can get up to zone 4/5 and still have some spare gears and a puddle of sweat on the floor.
As mentioned above, rollers will improve handling, balance and help you get miles in your legs. Turbo is better for intervals, threshold workouts.
Doing intervals on rollers could be a bit hairy! The real answer is get both!
I've used both but currently work with rollers. I prefer them, seems to give a more real cycle ride and smoother pedal stroke. I do intervals (ie 7x3 mins with rest) mostly but in the winter was riding up to 2 hours 15 mins on them.
They do take a bit of getting used to but helps you ride straight, can even use my aero bars and drink from bottle cage without to much fuss.
Would defo recommend rollers over turbo.
Cheers, rollers first and then buy a turbo later.......
of course - the triathlete's answer... why didn't i think of that...