to answer the question,
in my opinion, I would pay the extra for the bigger amp, 2dbs is not a huge difference, but it is a difference.
and not to continue the argument
I have always thought
double the power = 3dbs (not taking compression into account)
double the cone area = 3dbs
double the cone area, and double the power = 6dbs
so, adding a second sub with the same amp is 6dbs. This would be ideal and not actual, but it would be more than 3dbs.
And as far as clipping goes, I listen to music quite loud, and i know my amp clips. But to me its not audible until clipping is quite bad.
Find a rock song, that you know is quite loud in volume, if you open this up in a sound editing program, you will see lots of clipping. I was amazed the first time I saw this.
This will give you more overall spl, but less dynamics.
Same thing with clipping a sub amp, more overall spl, with the same max spl, and overall less dynamic.
I really think the average person clips their amps much more than they think.
I have never blown a sub, and am quite sure I clip my amp quite often. But I try to keep it down enough that it still sounds clean.
This again is all based on my view,
Anyways 2 dbs, will give you a little more headroom, and offer a more dynamic sub.
I would pay for the bigger amp, thats just me though.