Love soulseek but please do try and share some stuff and leave you computer on from time to time so that other people can search / download what you are sharing. Whilst it isn't a requirement and you won't get banned for not doing so it is basic etiquette in my opinion and without people sharing resources like these will slowly die.
In terms of clients I would suggest trying both out. The majority of people recommend nicotine+ but personally I don't understand it, I find the official client much better even though it has a few quirks mainly due to searching and navigating others files seems to be a lot better in my experience.
I've only played Borderlands 2 in local split screen co-op.