First off, if it is your second car. Do some bolt-ons and enjoy it. A lot of guys want to start throwing money at a car and then realize it's not what they were after. I would rather drive mine than watch it sit in the driveway cause I need another $200 part or something like that. Collect ALL of the necessary parts, before you turn the first wrench on your toy. The parts laying in the garage or the basement are a lot better than the parts AND the car! Drive it!!
I have built a few 351 Windsors. (302 and 351 are both Windsors. Built in Windsor, Canada.) Built alike, a 351 will pull the teeth out of a 302. All below 5000 rpm. (no expensive valvetrain parts.) All of the 302/5.0 heads work on either block. Although the 351 uses a 1/2" head bolt and the 302 uses a 3/8" (?) head bolt. No problem. Cams interchange. There are two different firing orders. No problem. Cams and heads that would be "too much" for a 302/5.0 would probably be "just right" for a 351. I would build a 351w in a minute. If you're into strokers, the 351 can be pushed out to 377/392/408/427 cid. I think someone put one over 450 cid, that's not within easy reach of most first time builders, though.
Get a roller block out of a pickup truck. Put the stroker parts in it, if you're so inclined. If not, it won't cost much more than a 5.0 to rebuild as a 351. Stroker parts should cost the same, either direction. I think that truck used a nice lower intake, so you could save a little $$$ there. There are a lot ways to do either one right. They are both Fords, after all. Hahahahahahahahaha!
351w/Mustang specific parts include headers (Cobra Mustang came w/ 351w), oil pan and pick-up(Jeg's sells these), flywheel(some aftermarket can be made to work either way) and some other minor things I can't think of, right now.
I would say to build the 351. You'll be glad you did.