Well, I've owned both. I had an SNJ-5 for 9 years and have just bought a T-28A.
I like both a great deal, and both have pros and cons, but respectfully I'll take a different position than some others.
Yes, I got an A model. If you do a bit of research though, you will find that almost all A models have now gone to a Hydromatic 3 blade and removing the original avionics, inverters and associated crud loses about 600 pounds improving performance. Putting these together, it adds up to a 170 - 180 KNOT cruise at 36 gallons per hour. A wee bit higher fuel burn than a T-6, but about the same miles per gallon and the extra speed and more comfortable cockpit makes X-C a lot nicer. As to the 'conventional wisdom' of sluggish performance, the T-28A I have will do at least as well as the SNJ did - maybe because of the lighter weight and 3 blade prop, but it just does. I've been there, seen it myself. It's reality.
Flame suit on here: The T-6 flies great with wonderful control responses (I had the fast aileron cranks). With 10+ years more of technology, the T-28 simply flies better: light control pressures and the most amazing balance I've ever flown save for some jets. There it is, they both fly great but the T-28 simply flies better. And, I don't need to prove myself anymore that I can fly a taildragger...

Maybe I'm just old.
Upkeep I do believe ON THE AVERAGE will be lower on a T-6; that said, a well sorted T-28 will cost less than a middle of the pack T-6 to maintain. The T-6 is just an older plane, has more corrosion problems and things just wearing out. The T-28 does have more things to break, though, especially if you keep the original electrical system (mine doesn't). Interestingly, the T-28A seems to have fewer ADs than a T-6.
So, I started out looking to buy another T-6 and ended up paying MORE money for a T-28A based on side-to-side fly-off comparison. Maybe I will change my mind later, I do not know. I think the A model is a much better buy than most BCD Trojans and with T-6 price escalations, a better value for the money than the T-6. It surprised me as I had all of the previously here stated preconceptions about the T-28 as well - however - when you fly them back-to-back and assess for yourself with talking to folks in the NATA objectively (who are the knowledge) the story changes dramatically in reality. Well, That's just me, and I put my hard earned money behind it.
If you are buying the airplane for the love of flying, you cannot go wrong with either.
YMMV