It really depends on your budget. If you buy a 7D and keep if for 5 years, great. If you buy a 7D and keep if for a year, then sell it and buy a 5DIII, maybe not so great. It's a bit different with good lenses, they hold value well (and sometimes gain value over time). Bodies lose value...fast. A used 7D can be had for $1K or less these days.