The 1x12 will likely sound tighter and more focused (possibly thicker depending on how you define it by your ear) the 2x12 as you suspect is likely going to sound more airy or roomier but perhaps not quite as focused.

Guys with mith more tech experience will likely weigh in on the wattage thing per speaker and my guess tell you that it aint gona matter too much in regards to the thickness of your sound.

OK - I own an original Spider Valve MkI 112 and a Spider Valve MkII 212.  As far as the power amp circuits go they are both identical or very nearly identical as far as I can see.  The 'front end' or pre-amp/DSP sections are different and it's these that give the amps their various flavours.

I have used my old SV 112 quite a bit live, and Karl is right about the power output - in my opinion the 40w tag is a very conservative estimate.  Power ratings alone do not a loud amp make, but the power rating does have its part to play along with speakers.

The SV112 always sounded great if I added a 1x12 extension cabinet loaded with a Celestion G12-75T - and it was easily as loud as the other guitarist's Marshall TSL100 and 4x12.   On its own with just the V30 speaker it still sounded good, but definitely not as full nor quite as perceptibly loud.

Two speakers will always spread the sound from the amp more widely and in my humble opinion two speakers is better than one.

Now on to the question of which sounds best - MkI or MkII?  I love the sound of my MkI but I have applied a couple of after market modifications in the form of an SVpre and I changed the speaker from the V30 to a Celestion G12-K100 mainly on recommendation from other users and the fact that I got a good deal on the new speaker.  The possibilities with the MkI SV are limited when compared to the SV MkII as there is no way to update the amp or to be able to interface it with a computer for preset editing.

The MkII on the other hand allows updating and computer based editing with the latest firmware (v1.50).  Opinion is somewhat divided on the sound of the MkII with firmware v1.50 installed as opposed to its predecessor v1.17.  Personally I feel that the tone of the amp and models was warmer with 1.17 than with 1.50, but the added FX and the ability to edit presets on computer outweighs any difference in tone (although I would like to see the 1.17 amp models with the added FX).   I'd still say the two speaker configuration is better in the SV MkII 212 than the 112 SV MkII for what it's worth.

I would seriously take your guitar along to your Line 6 stockist and try the 112 and 212 Spider Valve amps side by side before buying and see what you think of the sound yourself as it is such a subjective topic and only you can decide which you like best.

Nick