They both have a huge selection but the selection of the repos + AUR has many more applications that I needed than the void equivalents. Arch has a substantially larger user base that supports the AUR.
I didn’t notice any difference in stability. Both were/are stable for me. However, stability means different things to different people.
The primary reason I don’t use void is the lack of systemd.
On the other hand, void has a newer and more sophisticated package management system.