i wouldn't worry too much about building up a base of strength or stamina, you'll still be absolutely pooped after your first session and i'd wager your judo has you at a decent level of fitness. however if you don't feel comfortable sparring immediately then that's entirely your call.
i wouldn't worry too much about going somewhere as a training ground before you move on to another club. at the end of the day if they are a good club then they're a good club and you'll benefit from their teaching from the very start. they might have some serious fighters but everyone has to start somewhere and at the end of the day they want to get their beginners up to standard. the fact they've actually got some fighters sits well with me as well.
Try them both out and see which you prefer, but don't go to one intending it as an introduction and then move on to a 'better' club as it can be harder to unlearn bad habits than it is to just learn new skills.