The west side tends to get currents, being more open - Koh Tao really is more beginner friendly. Visibility on Tao in March can be hit or miss, but it doesn't matter as much for getting certified. Second the recommendation for Master Divers there; Elaine runs a great shop.
I don't think there is a specific spot for large pelagics; they can show up all over, but the chances are fairly slim. I did a bunch of dives on Koh Tao and never saw anything bigger than a turtle, but I know people who saw a whale shark there on their open water course. Personally, I have only seen whale sharks at Richelieu Rock up north and Eight Mile Rock down south (ir's named for being 8 miles away from Koh Lipe), while my only manta encounter thus far had been at Hin Daeng. I met a diver who had seen a whale shark at Stonehenge, which is a really shallow (6-8m) dive site near Koh Lipe - one would think that a place where a whale shark's tail can brush the bottom while its mouth pops out of the water would be the last place to encounter one, but there it was.
One possible option you may want to look at, depending on your time constraints and how much you end up liking diving, is a course at Koh Tao followed by a liveaboard in Andaman Sea. My last trip out (January 22-27) we had a lady onboard who had just completed open water on Koh Tao, and she had the time of her life at Richelieu Rock, although she did have some trouble at first with the more challenging diving conditions there. You can find her review at
TripAdvisor here.