D (HendersonD), the drawback to testing out a rear pannier only scenario with a new bike is that you may be convinced that the handling is good, esp if the bike is real step up from what you've used in the past. I assume the new bike has LowRider fork fittings. At the very least, borrow somebody's LowRider rack, put some small panniers on, load them with bricks or whatever and ride them. Go climb a hill so steep you have to stand. Do some fast traffic starts, out of the saddle.
Notice virtually every poster here who's tried the LowRider approach has adopted it. (4200 miles is a long ways to go with a 2nd best system.) Yea, it will cost you another $200 and add several pounds but, 4200 miles is plenty of time to appreciate a better ride. (And the weight? Being able to get out of the saddle when that weight matters is a real blessing!)