I am currently both UA PE and AA Plat. My current bias is that I like AA a lot and don't like UA much these days, so you can take what I say with that in mind. Why don't I like UA? Customer service, period. I won't go into UA-bashing here, but I'll give my observations about the statuses.

PHONE SERVICE: I rarely if ever have a hold time on the Platinum line. I sometimes have a hold time on the 1P line, and sometimes don't.

AIRPORT SERVICE: I feel that I get pretty good treatment at the Plat level. AA has Platinum Service Centers at hub airports, whereas UA only extends this service to 1K's. When I've encountered irregular ops, AA has been quick to find the best way home for me, and confirm the flights - even on other airlines. UA has led to some of my most disastrous experiences ever, and I've had to drive to my destination on more than one occasion.

UPGRADES: I almost always get to upgrade when I want to on AA. On UA, it is about 50-50. I realize this depends largely on your routes and the popularity of your flight (obviously, Friday afternoon flights are tough to upgrade, and I don't *always* get those on AA). Check the UA board to get a feel for how hard it is to upgrade from IAD - I remember a couple threads dedicated to exactly this topic.

E+ vs. MRTC: I think E+ and MRTC are about equal. Perhaps E+ has an inch or two more room on some planes. E+ seats go to any Premier member, or anyone who pays Y/B fare. Being a 1P doesn't give you any additional advantage over a 2P in terms of getting an E+ seat. I tend to prefer MRTC because I sometimes change flights at the last minute or fly standby. On UA, I probably wouldn't get E+ even with 1P status. On AA, I may be in 28B if I switch flights at the last minute, but at least I get the extra legroom...

ROUTE MAP: From DC, UA clearly holds the advantage over AA. It's up to you to determine how important this is...