EMasters, Super Audio Mastering, Loud Mastering, these are all online mastering houses the last three are located in the UK, the first Audio Source is US. You could also check out Laquer Channel (not sure on the spelling) they are here in Toronto. I've heard good things about them as well.
but just as a general rule. A mastering session will maximize your track to bring out as much of the track as possible without colouring* the sound of the sound. Also they'll EQ/Compress/Expand the track to best suit the medium it's intended for.... its a minor thing... but critical for a maintaining the artistic sound of the music no matter the medium.
CD's have a different audio dynamic then vinyl.
With vinyl one key issue is usually phasing. If you plan to cut to vinyl you REALLY need to keep everything in phase or else you run the risk of overloading the cutting head while making your master disk. Also other issues such as panning and sudden noises could also cause issues. Skips, are a main result, also loudness in audio level may be a factor.
As for mastering to CD, I dont know off hand the issues with it.. but they do have to be EQ'ed specifically to work with the limitations of that medium as well.
colouring - creating a sound in the final product not intended artistically