The 351 Cleveland-
Pro-It's a beast you have to do little to no work on the cylinder heads to get power, I would suggest using the the 2V heads on the street to make power, the 4V heads have way to big intake ports (you can nearly stick your fist in it).
Cons-The Cleveland does not have a huge aftermarket unlike the 351 Windsor, it has oiling problems, and it is difficult to find a block that has not already been messed with since the blocks are so rare. (I believe they were only produced from 1970-1977?)
The 351 Windsor
Pro-It has a huge aftermarket, its easy to find a good block, there are lots of books about how to mod them. They are very cheap
Cons-They suck big time when left stock
It all comes down to preference but in my opinion I would chose to build up a 351 Windsor just beacuse its cheaper and has a better aftermarket. Also if you want to stay original the 351 Cleveland was not offered in 1969, so if your restoring your mustang to original specs that rules it out.
Anyways have fun with your choice there both awesome engines.