@Daphoid
I have a 40 and a 100. Both are very good sounding amps. And are so close in size that I wouldn't worry about space or weight with either of them. I would say the main difference is going to be the effects loop. With the wattage being second. For me anyways. Some people claim to prefer the 10" over the 12" speaker also.
I've only had the 40 up full blast once and it gets quite loud. I use it all the time when jamming with a friend on just acoustic and singing and never have it up past 10-11 oclock, even with the gain all the way down. I don't think it'd cut it with drums, which is why I added the 100. Still waiting on a chance to pull that one out and use it!
Whichever one you go with im sure you'll be more than satisfied. These are great little amps. The "flat" tone on them is just about perfect, add the super lightweight and it doesn't get any better.
Here's my 40/100 stack.
I'll also add that if I had bought the 100 first, I would have no need for the 40 since they are similar in size/weight and the 100 can be turned down to whatever volume is needed. The headphone out sounds great too. I use that a lot for practicing while the wife's asleep.