For my salads I always chose firm tomatos. I like the liquid inside, but usually remove about half of it, otherwise the salad will be too watery.
First I cut the tomatoes in half and remove some seeds/liquid and then I either slice them very thinly (if I'm making a Greek type of salad of sliced tomatoes, sliced onions, feta cheese and some oregano), or cut them into wedges for a tossed salad.
Also, I try to cut down on the amount of vinegar or lemon depending on how much liquid I added from the tomatoes.
If you are making "pico de gallo" it's good to completely remove the seeds/water before chopping the tomatoes