I tried the latest nightly October 15 on my Nvidia Shield Portable Kitkat, and there are still three mame cores available: Mame (2003), Mame (2014), and Mame without any year. The Mame core works fabulous, the Mame (2003) and Mame (2014) show different errors. One of them shows the “Open archive as folder” option, the other one just crashes after selecting it.
Since the Mame core works just fine and seems quite fast on my Shield, I am pretty happy with it actually. Thank you for adding that third Mame core at some point. I understand that the Mame (2003) core might still be helpful, because it will probably run some games faster. I am not sure what the use of the Mame (2014) core would be if the Mame core is up-to-date.
The Mame core also supports the analog controls on the Shield Portable very nicely now! Kudos to the Retroarch team for supporting this. I remember in earlier versions the analog sticks were supported, but always treated as digital inputs only.
I also quickly looked at the speed of the Retroarch Mame core against the program Mame4droid 0.139u1. Result: Retroarch is much faster, almost a factor 1.5 on some games I think. I tested this with heavy games such as Outrunners, etc. By the way Outrunners works great in Retroarch on the Shield Portable now, which is an amazing feat for a handheld console! It is such a great game, too!