It's funny because I've gone from Mustang to WRX back to Mustang. Specifically, I had a 2000 Mustang GT, traded that in for an '04 WRX, then went to an '05 Legacy GT, and then to an '08 Mustang GT. I owned these for about 3 years before each trade (except the WRX, had for about a year and a half).
I really enjoyed all the cars, especially the Legacy GT. But the main complaint I had about the Subarus is the lack of torque compared to the Mustang. Obviously this is going to be the case when you're comparing a four cylinder to a V8. But I found it annoying that every time I had a friend or two get in the Subaru it felt like an entirely different car. The extra weight easily bogged down these vehicles. The Mustang, while I'm sure the extra 150-300lbs added from friends did affect performance, the car never really "felt" different.
Additionally, mods are abundant and cheap for the Mustang. I think I paid something like $500+ dollars for a Cobb ECU tuner for the Legacy GT and that same amount bought a tuner and cold-air intake for the Mustang.
Reliability-wise, I never had a problem with any of them...well, except the Legacy had some ECU issues that caused the radiator fan to always run, but that was it.
If I was looking to purchase a new vehicle it would definitely be a Ford or Subaru. It really depends on what your needs are. If you do not need the AWD and 4-doors, then I think the Mustang is a no brainer (I've seen V6s brand new go for under 20k and 5.0's for less than 26k). But if you live in a region where you get snow and/or have children, I think the WRX might be a better fit.