A woman at work has this friend who is very intelligent, a professor I think. Hand he told her that there is not real difference between a 3GS and the iPhone 5 and said its not worthy of an upgrade.
One of the girls at work has just replaced the screen on her old iPhone 3GS after she dropped it and it smashed. After seeing my iPhone 5 she now wishes she hadn't done this and got an iPhone 5 instead
I went from a 3GS to a 5. If you have the money, the 5 is obviously the superior phone in every way. If you are strapped for cash, the 3GS is still an awesome device. It will probably be end of life (no iOS 7 support) in a year though.
If it's money, I understand; but if it IS money, why would you even be CONSIDERING the 5? Do you already have the 3GS? If so, are you happy with it? If yes, then just KEEP IT. Use it 'til it dies.
If you're buying outright (no carrier contract), why not consider a 4 or 4S? I used a 3GS until just two weeks ago. My sister had to change carriers because her new job put her in a location where AT&T doesn't have signal. So I inherited her iPhone 4; now, I'm up for a upgrade in November, but I'm not sure I'm going to do it. The 4 is such a huge upgrade from my 3GS, I'm just thrilled with it. I may save my $200 and wait for the 5S (or 6, or whatever).
If you're talking about a carrier contract, uh... the 3GS isn't sold any more. And the 4 is FREE...
iPhone 4's are $200 (or so) in good used condition; 4S's are in the $325-ish range. 5's... well... you better bring several hundred dollar bills...