CessnaDriver,
If you already have the UPT slot, to go Guard Reserve now, you will be known as the guy who dropped an active duty UPT slot. The recruiters called you back to get you what you requested, they followed through because you were a good candidate. Your reputation will precede you to any Guard/Reserve unit. You certainly have filled out alot of paperwork that lets everyone know you were going for the pilot option. It will pop up. You will have to answer the question. That will likely get you turned down for a Guard slot. A question of conviction.
IF you did not have the UPT slot, I would recommend the Guard option as a very good move. However, you just got the invite. IMO, bit late.
Certainly that is just my opinion, but sitting on pilot selection boards in the past in reserve world, working joint duty now, I know the breakpoints. Besides which, as pointed out, the AD folks looking for Guard/Reserve slots are plentiful right now with airlines taking the cyclical downturn.
Summary: Go after Guard/Reserve with great caution with UPT slot in hand. No guarantees. Best of luck with the decision. Either way, USERRA protects the Mesaba job till all obligations complete.