I agree with brett, take both. If you're doing alot of shooting the 20 would be easier on the shoulder. But if you aren't on, having an off day, etc, switch to the 12 gauge.
I've spent most of my shooting clays with a 20 in my hand, and have broke alot more clays with a 20 than I probably ever will with a 12. If I'm on the 20 is all I need. But there are days where I just can't get down the lead or the timing. The 12 helps, but some days nothing works...
later,
scruffy