The sweet spot for 3" IMO is carry. It is smaller than a 4" and OAL on a revolver, even a J-frame, is usually longer than a comparable auto to begin with. It is a very handy length, gives more velocity and thus more reliable HP expansion than a 2", will have less muzzle flip than a 2", etc. The Python is a bit large for that. So, from a purely practical standpoint, I'd say the 4" would make more sense, it is longer so you'll get even more velocity out of the round and thus more power and more reliable expansion, and a longer sight radius for easier practical accuracy. However, there is something about a 3" revolver. I doubt I'll carry my 3" S&W 65LS more than once or twice in the next 5 years (too expensive to replace, in too nice cosmetic condition for me to feel comfortable carrying it), and I bought it years before I had any carry permit, and yet, it is one of my favorite guns. I most definitely want a 3" S&W 686+ even though I doubt I'd carry something that size very often. So, if you are someone who appreciates a 3" revolver, you probably need the 3". If you aren't already in that camp, from a practical perspective, the 4" probably makes more sense.