Of your two choices, I would definitely recommend the M4. AR Style rifles are easy to find, easy to use, and externaly the most customizable, in my opinion. There are parts out the wazoo for AR's. For a manufacturer, the King Arms M4A1 is a great choice. Full metal body, with full trades, and a great performing gearbox, all for around $300.
Primary: KWA LM4 PTR, Classic Army LWRC M6 A2, Classic Army G36E, AGM/Javelin MP44.
Secondary: KWA Mk. 4 1911, KWA Mac 11, Skeleton Sidearms 8" .357 revolver.