Thank you, Landric. From your posts, I know you have a nice collection. Do you find yourself using the M11a1 often? For the expense of an extra inch or two on the rear end, it seems like the M11/9 opens up many options (particularly the various Lage offerings), but there is a charm I can’t deny for having a machine pistol that is not too much larger than a common carry pistol (to be clear, I have no plans to actually carry the gun; it’s mostly an emotional affinity).
Keep in mind that I don't have a M11/9, so I can't offer an opinion between it and the M11A1. I have three MACs, the above M11A1, a M10/45, and a M10/9 with an Uzi mag well conversion. Of the three the M11A1 is my favorite, though they all have their advantages. I really like how small the M11A1 can be, but it can also be nearly as large as a M10 (but much lighter) with the right upper and stock.
I think the advantage the M11/9 has over the M11A1 and the M10 has started to disappear. Lage doesn't make their .22 LR kit for the M11/9 anymore. On the other hand they offer the MAX-31 and MAX-15 uppers for all three sizes of MACs. There are still a few more Lage options for the M11/9, but it isn't like it once was. Sam at PS also offers basically everything they offer for the M11/9 in a version for the M11A1 also. As best as I can tell the M11A1 is still on average $1-2k cheaper than the M11/9.
I haven't tried the M11A1 with a 5.56 upper, my MAX-15 upper is for the M10. I suspect the M11A1 version is much handier. When I have a spare $3k lying around I plan to get a MAX-15 for my M11A1. Even so, the M11A1 is my favorite of my three MACs and is the one I use the most. If I could only keep one that would be it.
I would absolutely buy the M11A1 again. If you get one I think you will be very happy.