It depends. The RAR5 format used by newer WinRAR versions (the “old” one is still supported just well) can have smaller archives than 7z, but the opposite is also true. Still, yes, WinRAR is in my experience faster and more stable.
(Note that “as small as possible” is not usually the most relevant point. The best compression is currently reached with the ZPAQ format, but using it with maximum compression settings is painfully slow.)