NSIS is ridiculously small and for the most part very powerful. One major flaw is having to manually add a line in the script for every single last bloody file you wish to remove during uninstall. There's no way, afaik, to remove an entire folder or better yet, for it to automatically remove all the files that were added with some sort of flag. It's an annoying oversight when your program has over hundred files. You can of course write a script in PHP or C or something that generates the NSIS uninstall data. Otherwise I love NSIS and highly recommend using it. Ever since they switched to the modern GUI there's no longer any aesthetic reason to not use it. Note I've never used Inno so I have no idea how it compares.
I started with NSIS in 10 minutes. There are dozens of easy-to-full-featured examples on the net and I quickly composed quite a good script (quick without components). Footprint size is awesome, now my bottleneck is Audiere/DUMB support for MO3/OXM. My 1st April game is ~113Kb, Flash wrapper itself adds ~700Kb, music is almost 1Mb but my friend will fix last one soon. So entire installer is less than 900Kb without music. That's the footprint for good Flash wrapper and NSIS