My personal preference is .mkv with .ssa softsubs. This allows for the subs to be styled by the group, including whatever fonts and colors they need to, as well as basic effects.
However, if you're not going to style anything special, or don't have any spiffy fonts that you think everyone should be seeing the episode in, I'd suggest just a simple .srt.
As for the ogm vs mkv thing, I'd personally go mkv, since I've talked to more people that have had trouble getting ogg to work than I have people that had trouble with .mkv. It also seems like mkvs seek a bit faster.
That being said, it's really all your choice, whatever you feel comfortable/happy with.
