Well, I have one MP5 set up old school with the slim forearm and another with the KAC RAS. In general I am not a big fan of picatinny forearms because I don't like how they feel. Much of this can be remedied with rail covers. I find the HK style covers and even the KAC covers a bit bulky bu the rubber/plastic ladder covers work well. The beauty of the RAS is that you can attach a vertical fore grip, lighting, a laser, etc so they are pretty handy. I also have a two point sling with quick detach buttons and the front button goes on the RAS. So, overall, I think they are pretty handy. AND available for reasonable coin now that KAC has made a new batch.
Good luck and let us know what you end up with. And pics are always a must:)