TBH, The speaker may make a bit of a difference to the tone - but if you have a decent amp then you won't be disappointed with slightly cheaper speakers...
Also, my Screamer combo sounds immense with its stock speaker (one of the G-somethings? G-12 maybe...but that could be wrong). Admittedly, it might well be drastically improved with a better speaker, but considering the price, I'm not disatisfied with the tone enough to warrant the money
I also do not regret getting a 2x12 as I was originally looking at - but it is plenty loud, and can fill a room on its own. At some stage, I may get another 1x12, so that I can position one on either side of the stage and get better spread - but 2 speakers next to each other is overkill to my mind :P