It depends on the price difference and how keen you are to fiddle with settings.
3350P obviously has no built in graphics but I'm assuming you already know that.
The 3450 and 3350P have the same stock speed but the 3450 has a better turbo mode. If you are just wanting to plug something in and not worry about it, and the price difference is negligible, then the 3450 (or 3470, its usually more or less the same price) is the one to get.
If you have a higher end motherboard though, you can fiddle with settings and do a basic overclock by fiddling with turbo settings (there is quite a good example of this in the Q1 $600 budget build on the main page)and you are unlikely to see much difference.
In the US I'm showing the price difference as about $10 between 3350P and 3470, here in the UK it's about £3. Personally, I'd get the 3470 if that's the kind of price differences you are talking about.