There's no better as it's just the space difference. Like many forumers advice, if your budget is tight, get a 8gb one. But if you have extra budget, go for a 16gb.

But again, it all depends on what you wanna do with your phone.

Do you only make phone calls and text from your phone?
Or do you play games?
Or it's the Wifi feature that you're looking for in an iPhone.
Or because you want it for its iPod feature?

If it's only the phone, text and Wifi feature that you are looking for, 8gb will be more than enough.
But if you're more to the gamer side, a 16gb will be more advisable.

I got myself a 16gb one because I really make full use of my phone.

As a portfolio for my photography jobs, music, videos and games to keep me entertain while waiting for client.



So bottom line is you'll have to decide what you wanna do with your phone. From there you'll know the answer of which to get.