Depends what preconceptions you have on each on them both. Most people say Perf Tuning is one of the hardest. Id say they were pretty much on a par. But if I was making a choice, I'd take Perf Tuning, just get it out of the way. When you pass that, it should fill you with enough confidence to easily pass the SQL exam not to long after.
I haven't taken P&T yet, but if you've got quite a bit of experience with SQL syntax, I'd do the PL/SQL first. To prep for that one I just used Couchman's book and the sample test. I don't think it was very hard, mostly a lot of syntax questions. I've heard P&T focuses a lot on the V$ views....what's in each one, which one to query when, etc..
This all depends on the individual. For me PL/SQL was a living hell, I failed it twice. Prior to that I had no programming experience so that is one reason. I did have Oracle installed at home and practiced my butt off each time I took it.
Oddly enough as hard as Perf. Tun. was it was that exam that I scored highest on, I was surprised!
So, my vote is get pl/sql out of the way, but be afraid, be very afraid. Bwoo ha ha ha harrrrrrrrrrr