I have the i5 and my wife still has the 4s. I have fooled around with both and in some regards I still sort of like the 4s because it's more compact and my small fingers can easily reach all aspects of the screen. But for apps and other things, I definitely like the slightly larger screen. We both have the Otterbox Defender around our phones so that of course adds some bulk. If I were to use the phone without the case, a bumper or slim case, I would probably tell a huge difference and my views would be definitely for the i5. Compared to the 4s without a case it definitely feels much more comfortable in hand. Even with our Defenders covering each, the i5 feels like a feather compared to the 4s. If you are weighing which one to get and don't care too much about having the newest thing out I would probably go for the 4s. Because IOS6 is available it really acts and operates, for the most part, the exact same as the 5 with slight differences.
If you want 4G LTE then the iPhone 5 would be the choice. As far as size differences...the only difference is in length not width so that is the only thing about the iPhone 5 that will remotely feel bigger. Weight wise...the iPhone 5 is noticeably lighter than the 4s but in no way is the 4s a brick. If you never put your hands on the iPhone 5 you would never imagine the 4s as being considered heavy lol.
Personally, although I have no problem recommending the iPhone 4s, I would suggest the iPhone 5 unless you were restricted in the price you wanted to pay for the device. Neither is better or worse...it's just that some features are not the same and if you were ever interested in an iPhone with 4G capabilities then there is only one choice (personally, 4G is nice BUT it's not something I feel like changed my service or enjoyment of the iPhone so it's not a deal breaker. I could have done with or without it). Oh...and the front camera is finally a little better than the one on the 4s (and the 4)...probably one of the more pleasant things in my opinion when wanting to use the front camera for a quick pic lol.