I won't get into the Glock vs Sig vs CZ vs 1911 debate, since I'm sure it's older than most of us. However, having said that:
What do you want to get out of it? I like the style of the ISSC, and I suspect you can get high capacity magazines for it, since it is a rifle. The Glock conversion, on the other hand, is still considered a pistol, so you're limited to 10 round magazines even in a .22 caliber. The ISSC is a completely separate gun, which may be good or bad. On the one hand, you have two guns, so you can take someone shooting with you, or you can easily sell one if you choose to upgrade to something different. On the other hand, you now have to maintain two guns, meaning keeping track of two different sets of parts (although you can always shoot one if the other needs work). Finally (and this is probably the big difference), one is a pistol and the other is a rifle. If you want a rifle to play around with, then get the ISSC. If you prefer the handgun, then get the conversion.