Reserve/Guard
The only information I can offer, as a reservist, that might make me choose the reserves over the guard is that there are many TDY opportunities the reserves are offered that the guard are not. The 3 week planner jobs in Germany, the 2 week rotes to here/there, etc. A buddy was going to apply to a C130 guard unit and he asked them about those TDYs - he then called AF Res Command - he was told that many are TDY billets the AD can't or do not want to fill. They are given to the reserves and left "open" for a certain period of time. If they are not filled, the guard gets a quick cut and them, then they go right back to AD. If they are filled, the guard never knows about them.
Other than that - from what I hear from my buds the guard is an overall better environment. The reserves, in this day and age, can be the active duty "B" team-i.e. - being called to support active duty more than in the past. I don't know if you want that or not. The guard has the state Gov to say yeah/nay.
Good luck!