I don't have experience with either, but a quick Google search shows that the WRX is rated higher than the GTS. I personally would go with the GTS simply because age (along with mileage) is what kills vehicles, and that WRX is already 13 years old, whereas the GTS is 8 years old. That being said though, I think the WRX is going to be faster.

A 2003 WRX is 3,085 to 3,220 LBS and makes 227 HP. That means there is 1 hp per 13.6 to 14.2 LBS if that makes sense. The GTS is 3032 LBS, and makes 143 to 152 HP which means 1 hp per 20 LBS at best. The reason I look at this is because the cars weight is what determines how fast it goes. Your car might make 10k HP, but if it weight 200k LBS, it's not going that fast. When looking at the HP to weight ratio, you want that weight number as small as possible.

All in all, if you want speed and (arguably) reliability, go WRX. If you want a newer (again arguably) more reliable, better looking car, I'd go with the GTS.

The 2007 Mustang GT weighs 3,483 to 3,601 LBS and makes 300 HP, so 1 HP per 11.6 to 12 LBS. The California Special if I remember right is just a body kit, no power plant mods.

The 2013 MazdaSPEED 3 weighs 3,281 LBS, and makes 263 HP, so 1 hp per 12.5 LBS.

Taking JUST weight and HP into this equation, both friends will beat both cars you listed. HOWEVER, there are more things that go into it, such as gear ratio in the diff, extra weight (any tools, the driver, etc), and most importantly, how aggressive the driver is. Also don't be so set on a manual, do some research before you jump into one. In the later years, automatics are getting better speed, better MPG, and tighter number groups (0-60, 1/4 mile, etc). The auto's are A LOT better than manuals in later years, so figure out what years for what vehicle an auto would be better than a manual, and the other way around. If you need a stick to keep your man card, go ahead, but people like me don't like to play with sticks if you know what I mean

Id avoid the GTS since they're basically a stock Lancer with a NA 4cyl and FWD. A WRX is a great platform for a quick build. With a few mods you can make it pretty fast that should be able to keep up with the mustang and speed3. Unfortunately the WRX is one of the most expensive cars to insure at a young age. I'd recommend an 05 Legacy GT (wagon or sedan). They're faster than the WRX of those years and can be found in manual or auto. A B5 Audi S4 would be another great choice and is the fastest of the 3. Making around 300hp with a bi-turbo v6 and AWD. It will be expensive to repair but certainly a fun car.