There are two differences:

One, stuff is a positive number. If your goal is to have less stuff to be happy, you can't get less than zero stuff. It's self-limiting. If your goal is to have more stuff, there is no limit. It's always possible to have more stuff.

Second, the OP seems to indicate that having stuff wasn't actually making him happy, which is an important distinction. Actually being happy is totally distinct from feeling like you will be happy if you buy more things.

If you are actually happy because you own things, then obviously you should stick with what works for you.