What is your question? Are you asking which amp is better? I'm not a fan of Alpine amps (overrated), so I'd go with 2 250G4s. If you look at that amp with 4 gauge power plug kit, it has 2 40A fuses. So you know with an unregulated PS, you will get much more than 800w rms in your car.
The size will depend on the type of music you listen to. For ultra low bass, 18" is better, but for higher bass note, above 35Hz, 15 will do just fine. Probably perform better too. From my testing, smaller sized subs responded and produced slightly more accurate bass. If your music consists mainly rap (50Hz-60Hz), then 15" or even a pair of 12" would do.
a better idea is get the 18 inch Mt, depending on the car, get that, then get it in quad voice coil at 4 ohms a coil instead of 1 or 2 or 3 or anhything, then get The Interfire IFB-2340dB amp, its a 4 channel amp, bridge the rear two channels into one coil, the front two into another coil and do it with another of the same amp, that should get it going