PM sent. Technically there are small and probably not relevant differences. The 720p subs are smaller and might work better with a space-limited format like BD25. Also some players may have bugs and either not scale the subtitles at all, or scale them improperly (the BDSup2Sub utility scales semitransparency badly, but I haven’t seen any bugs in a player yet). 720p subtitles don’t scale up to 4K as nicely as 1080p ones. But for most purposes, no – it should not matter. I’d match resolutions to the video just to be 100% certain there won’t be issues, but chances are you won’t see issues either way.