Are .smi or .srt files more compatible with the S3? I've been using .srt files exclusively on my phone thinking they must be the most compatible as all the apps that use sub files generally say use the .srt file format but today I came across an app that chose .smi files as the only option so I was wondering whether the "sub-community" in the Android world has moved toward .smi?  Now on an Android phone which type of sub file is easily converted to another sub format? And, if one is stuck just using a phone which of the two formats are much more flexible?

OS: 4.1
S3
This question is probably opinion-based. 

I'm not familiar with the trend in subtitles, but I use them a lot since I am hard-of-hearing. Most of the subtitles I get from SubScene are .srt files. Occassionally, I'll encounter a .sub and .idx combo, but RARELY an .smi.

A lot of media players support .srt; a lot others also support other formats. My guess on the app that you found which supports only .smi - which you didn't even mention its name - is that it may have been a niche app to play videos that have .smi subs. There may have been no other app that played .smi when it was made. I am just speculating.

But one thing for sure, .srt is not going away any time soon.