Your original question was: "Which do you like and why? I am thinking on a 27-30 footer. Also, is it practical (or desirable) to add wheel steering to a tiller equipped boat?"
The real question should be which one YOU prefer. By your question, I'd deduce you'd prefer to have a wheel over a tiller. But, it's not likely you'll be able to find many boats in the 27-30 foot range with a wheel. I, like others on this thread, think it would be more trouble than it's worth to try to convert a boat from one to the other. On a boat the size you're looking at, a tiller really does make more sense...that's why most have them.
I think, once you get used to the tiller (after one or two day-sails...and maybe even within minutes from the dock), you'll like steering a smaller boat by tiller better than if it had a wheel. A tiller makes the boat much more responsive which is what you want on a smaller boat. (The responsiveness is why you see tillers on even some large race-boats.)
Instead of spending the money on converting over to wheel, spend it on a tiller autopilot!
By the way, I sail a J/36 with wheel steering, 1 1/2 turn lock to lock which moves a balanced spade rudder within a 90-degree arc (3/4 turn = 45 degrees each side). Not being able to "feel" the boat/water/wind is total bovine feces. No, I wouldn't trade for a tiller on my boat. However, I've sailed on several smaller boats with tiller steering and I'm sure they benefited from it, and I know it would have been a disaster if they'd been wheel steered. On a larger boat, especially a cruiser, a wheel makes more sense.
What I'm saying is don't be totally turned off by tiller steering. If you give it a try, I think you'll agree that it's the way to go on a boat the size you're looking at.
In any case, I wish you the best of luck in finding the perfect boat!
Skipper, J/36 "Zero Tolerance"