your going to get alot of the same answers. Everyone is going to tell you to read up and search the board. There is plenty of information answering your questions. Im sorry if this has nothing to do with your topic but i thought i would just warn you. Here is a page on upgrading. http://airsoftcanada.com/showthread.php?t=2697
The m4 comes with a flat top rail to mount optics on. I dont know about the M24. Alot of people will say that since your a starter you probably wont want a sniper. Some people will say TM for reliability. I will leave the rest for more experienced people.
sorry for double post, my mistake.