Should I go with 5.1 "expanded" or 7.1?

[brettallica]

I am looking to get a set of tower speakers for the front left and right for my home theater. The current slimline Sony speakers I have are nice, but like so many of us on this forum, I've got a severe case of upgraditis.

My current setup is as follows:

5.1
Two Fronts
One Center (front)
Two surrounds
One Sub

To get an idea what my current setup looks like, click here.

So I've got a question as to how I should wire my new speakers. My receiver has the ability to have four speakers for the front (two for A and two for B), but it also has the ability to go full 7.1. Since I will be getting two new speakers, I will now have seven total satellite speakers. The question is: should I wire them in 5.1 and have four speakers in the front channel, or go with the full 7.1 setup? What do some of you audio experts think?

The 7.1 setup would look like this:

7.1
Two Fronts
One Center (front)
Two surrounds
Two surround back
One Sub

The optional 5.1 would look like this:

5.1
Four Fronts
One Center (front)
Two surrounds
One Sub

Yet another optional method for 5.1 wiring (not really considered in my original paragraph above) would be something like this:

5.1
Four Fronts
One Center (front)
Two surround rear
One Sub

I'm assuming this configuration would not be a good way to go, but I included it anyway, as it is an option after all.

Here's an animated image in case my wording isn't clear:

I'm leaning toward 7.1, but I want what's best. My room is about 18' x 18' or so. The rear speakers are behind/next to my sofa, where I do most of my HDTV watching.