The 302 is going to be around for a really long time. They produced millions of those motors over the years so I doubt that we will see the last 302 for a long time. If you want to get a 351 then do it and put the 302 on a stand in the garage and cover it up and keep it for later.
I could see not getting rid of the 302 if someone was keeping their 94/95 stang stock and in mint condition. If you start with mods, no reason not to bore the 302 or go with the much drooled over 351. Like said above, the 302 will be around for a long time.
I figure that car collectors and those who use 94/95's for show cars are the ones keeping lots of them stock and pristine. Who can argue with an 94/95 that has a 351? Saleen used them for several years in the S351 model. The beauty of the 351 is that it is a Ford block and its got a lot more cubes to make more power and torque. The 351 is the equalizer for SN95's to match GM's muscle cars. Of course, most who transition to the 351 also add many upgrades and mods that really increase the power output to a new level.
I'd keep the 302 around, but wouldn't think twice about going to a 351. I'm not sure what the difference in price is when considering the transition to a stout 351 compared to a total 302 rebuild with h/c/i, but I'd think it would be worth the extra bucks to do it right with the 351. I've got emissions to think about, so the thought of replacing my engine is mind boggling since I have to be careful not to waste money on parts that would kill my emissions.
Good luck. I'm interested in others opinions, too.