My apps are business related and sounds tend to annoy people, but on my personal phone I use M4A for everything...even ringtones. It is built into Android, so shouldn't give any problems. I'm not sure of load time differences. It is a little more complicated file structure, but it is also complicated to allow better audio frame sizes/windows, better compression, and more capabilities. I use 3-5 different Android media players playing both MP3 and M4A and I notice no difference in how fast they load. Now how they sound is another story...M4A sounds so much better and at lower bit rates has much less metallic sounds.
My big thing I usually run into is Constant vs Variable bit rates. I prefer Variable so I can use a higher bit rate with better sound and it can use less when needed. It doesn't matter if MP3 or M4A, but many players just don't do variable well still. I have my entire collection of music on the hard drive built in to my van's radio that claims to play them and it always cuts off the last 2-6sec of the song. Same thing with Windows Media Player and Sound Blaster stuff. WinAmp and a couple others play them perfectly as does Android and all the media players I've used on it so far.
When I made my CD collection to MP3 a couple years ago I went VBR at 196 bitrate figuring from my early MP3 days where everyone went 128 I'd get better sound. Turns out most CD audio needs about 256 to cover what the human ear can hear though. I just re-encoded the entire collection a few months ago and in comparing 256VBR MP3 to the max setting on M4A the M4A file was the same size or smaller in most cases and sounded better. So, I converted everything to it.