Madbulls are the best shells and as far as I know will work in most launchers (all the good ones), including the two you mentioned. I have one of those solid slug shells you mentioned, I've found it pretty much useless. It has very minimal range and is hard to aim, and if you lose it in the brush good luck finding it. Yeah, you can buy replacement heads for 3 for $20 but I still don't think it's worth it. If you want to try it out, just buy one to test first.
Also, one thing to keep in mind about these grenades: If you're going to want to use them in open fields you're going to want the co2 powered ones. They're a fair bit more expensive but the propane powered version has a max range of like 40' and that's with alot of arcing. The effective range is more like half that. Great for room clearing and CQB since it doesn't have a minimum engagement distance but not too usefull for large open ranges. The co2 ones can be powered with propane if you want to power them down but the propane one's can't be powered up. I personally bought the propane ones since they were considerably cheaper and I didn't need the co2 gas adapter ($70) and I really only wanted them for room clearing. If I had my time back I might have gone with the co2 versons. The cost is what's going to kill you. Buying the launcher is nothing, the grenades are the money sink. I bought six of the madbull propane 120rds and it cost me about $500. Six co2 ones would cost about a hundred dollars more then that, not including the co2 gas adapter.
This is a good thread to read about launchers, grenades and tactics: http://www.airsoftcanada.com/showthread.php?t=75325