Assuming it is a new model, neither. You should get at least 16 or 24 speed. Modern/new 7 speed wheel is likely to be freewheel and therefore weaker and unupgradeable. 8 speed and above wheel is likely to be freehub and therefore much more option for replacement and upgrade.
If you are looking at a vintage bike, 21 speed with a 7 speed freehub is good. No harm having more gears than needed. Never regretted having all triples in my bikes as my location is hilly and i very often have to carry load.