Normally, I would say to get the 40 as a true practice amp. But in your situation, with such a close price, get the 100.
The 100 is only four pounds heavier, two inches taller, and two inches deeper. But it is usually $100 more (50 percent more), which is significantly more money if you are shopping in the sub-$200 range.
I got the 40, but the 100 was never in the running for me. I was looking at 15 vs. 25 vs. 40. I wanted something very small, but still "bassy" sounding, to quietly noodle through when not in my studio (and for playing quiet electric bass along with fully acoustic instruments). The 15 and 25 didn't cut it for me. They have no low end, and the 40-and-up's e.q. controls are superior. I didn't consider the 100 because I liked the $200 and under price point for my intended use. If money had been no consideration at the time, I would have got the 100.
I have pushed the 40 to the max for gigs before, and it barely worked (not well) against a drum kit and two electric guitars (not played that loudly in a small bar). If I wanted an amp I could sometimes gig with a drummer, I would go for the 100. If I truly just wanted a practice amp, I'd get the 40. The slightly smaller dimensions, weight, and cost are preferable for a true kick-around bedroom amp, IMO. But as I said, your situation is different, because you appear to have a good deal on the 100 (either that or the 40 is overpriced there compared to here).