I would stick to the pitching wedge.
If you feel that you need a sand wedge try that. But, as you said, you're a beginner that plays a couple times a month. So just use what you have, and you will teach yourself to use it well. So in the end I don't think it would make much difference to you.
When I was in HS, I worked at a country club. My friends and I would go there once a week after school and play, and I found that I never needed more than a PW to get a decent score.
Once your technique improves and you start relying more on the backspin of the ball, then you might want to look into other wedges.
Also don't underestimate the importance of your driver and the other woods. Your chipping and putting don't mean crap if you can't get to the green.