I just spent the day on the phone with ASUS discussing this exact thing. Here it is...
IF, you signed up for the MGIB, that is 36 months, must be used within 10 years of ETS and is non-transferrable.
P911, costs nothing, is set in a percentage basis based on TIS. Up to the point of being in 24 months, P911 does not start counting until after you have completed your initial training (BCT/AIT or OSUT). At 90 days past this point you have 40%. This increases by 10% for every 6 months you serve. Once you hit the 24 month level (80%), then all time in service counts, including initial training. P911 is also 36 months, it can be transferred, but this incurs an additional 4 year re-enlistment.
What few know is that you can actually increase the 36 months to 48. You do this by first COMPLETELY using your MGIB, you can then add in up to 12 months of P911 benefits. You must have served 4 years to do this. Whether MGIB or P911 is better for you is controlled by numerous conditions that are best served by your career/education counselor or by calling ASUS at Ft. Knox.
At this point, and this could change overnight, the $600 add-in to the MGIB, which adds an additional $5,400 to the total amount is still in effect. Wouldn't count on this staying though.
Another area to definitely consider is using TA to go for your degree while you are in. This doesn't use any of your GI Bill (although you could, but it wouldn't be a good idea). You are responsible for books, fees, etc, but this is a super cheap way to either earn a degree or get a big jump. TA presently pays $250 per semester credit hour, to a max of $4,500 per year. While you may not have enough time to complete the degree during an enlistment, it can give you a huge jump start. Also remember that the enlisted TA does not allow you to duplicate degrees, you must always be going up...AA - BA - MA, plus certificate programs. It maxes out at MA, but there are also some MFA programs that qualify. An MFA is considered a terminal degree, like a PhD. If you have a BA, you can't get another bachelors using TA. No guarantee it will be around forever, but use it while you can.