Haha, I'm asking the same exact thing! Only, I have a dormant account with a couple of level 80's on it. Looking to get back into MMORPG for a fun, cheap timekiller. Plus, I'm currently building a new PC.
Rift has gotten mixed reviews, seems good, not great; but it has potential. Do you have a PC that will run Rift? Because running WoW requires less system requirements. Blizzard is interested in maintaining and building the biggest player base possible, so their games are usually more accessable for people with lower-end PC's. The computer I currently have, a lowly pentium 4 with 2 gb of ram and a 3 year old GPU was running the last WoW xpac at reasonable rates. Well, except for in Dalaran...
Also, Rift is a bit of a gamble, as are most new MMO titles these days. WoW has a massive community, and it's not going anywhere any time soon, no matter what anyone else says. Blizzard is solid, and there's a reason that WoW is the most popular MMO by leaps and bounds.
But again, Rift seems like it has a lot of potential. I guess the question is for you, do you have a PC that will run Rift, and are you willing to take the chance that most of your player base will disappear within a year if the game doesn't succeed?
Personally, I want to see more people play Rift so something other than World of Warcraft builds up a player base, and builds some reasonable competition thus forcing Blizzard to make an even better game.
So, to sum it up.
Play Rift because the graphics are better, it's newer, fresher and it builds on WoW's legacy.
Play World of Warcraft because it's accessable, there's a massive community with tons of great people in it, and it'll run on your 4 year old PC just fine!