You could do either. I couldn't believe it when a forum member (who's opinion I respect) told me he just plops in a garden hose and let's it rip. Me, being the adventurous (and lazy) fellow I am had to try it. I did a 50% water change from an outside faucet, cold as hell...and nothing happened. Fish actually seemed to enjoy it.
Now, I kept an eye on things. I didn't turn on the hose full blast or anything, more of a slow trickle. The temp in my tank normally drops 10 degrees Fahrenheit, 15 on a bad day. I try, with the speed of the incoming water, to keep the temperature delta at 10 degrees. This is my method on a 50% water change.
Recently I have stopped doing 50% water changes weekly (just because for my setup I don't think it benefits anything) and started doing weekly and/or bi weekly 25% water changes.
So my advice, take it for what you will, is on 25% changes no worries, run it cold...the fish will be fine. If your a 50% guy, then go slow or maybe look into the heater idea.
On another note, I don't know how much an inline heater of some sort is really going to change the temp of incoming cold water via plumbing. For your situation (assuming you still have that sump with 600 watts in it) I'd set it up to pour into the sump before the heaters and keep the incoming flow reasonable.