Noob needs help choosing

Hello,

 Total newbie here, be gentle with me. I will start with the basics of what I currently have.

Lexus Soarer (otherwise known as SC300/400) 2.5 twin turbo (Japanese only market).

I currently have this installed in her:

Alpine CDA-9812RB head unit (currently powering the stock 4" and 1" tweet fronts,also the 4" rear deck speakers)

JL Audio e2150 amp (powering the sub only)

Diamond audio M3 10" sub

Now I have recently bought:

Diamond Audio Hex series 5 1/4" components (to replace front door speakers)

Fusion - Powerplant speakers 4in 2-Way 120W - FPS-42  (to replace the rear shelf speakers)

My question is this:

I want to get a nice 4 ch amp to power the fronts/rears and I have 2 choices in mind which are:

Rockford Fosgate Power 851X

The question would be which has the features that you want?

Which has a better signal to noise ratio. The higher the better.

Which has better headroom?

The next question would be which will fit in the area that you plan on putting it?

The RF has a 100db signal to noise ratio

The soundstream has a 115bd signal to noise ratio

I suppose the RF would fit better as it is a bit smaller, it is also cheaper so that is a plus.

I guess I really dont need the soundstream, I just thought the extra power and looks of it was nice.