Me too.I bought a 9x19 Shield. It is more than adequate for me.
Personally I would try to shoot both first. If you have larger hands The Shield is a little tough to shoot at first. It just takes some getting used to. I would expect the 40 version to be a little tougher to shoot well then the 9mm version but I have not shot the 40 version so I don't know that for an absolute fact.
My primary criteria was I already have several other nines but I don't have any 40s and I really didn't want another caliber.