Where are you located? I have used both Blue Moon Camera and The Darkroom. Some services (mostly drug stores that send film out for processing) do not return the negatives. Check this if getting the negatives back is important to you.
C41 or B&W depends on what you want to do with your images. C41 is easier to scan if your scanner support infra-red dust and scratch removal.
Speaking of scanners, 35mm used scanners are pretty cheap on ebay. If you plan to scan your negs, this will likely work out to be cheaper in the long run.
Why not develop the negatives yourself? It doesn't cost alot for the equipment and you don't need a darkroom, only a changing bag.
Of the Kodak and Ilford films I have tried, my favorites are FP4+ and TriX in B&W and Portra for color negative film. I haven't shot much slide film.
Have fun!
Regards,
Rob