I would suggest three ways to go, two of which involve forming some kind of opinion: 1) Decide which speakers sound most enjoyable to you, using the music you most enjoy and will play most often, played at the volumes you will typically favor, and positioning yourself anywhere and everywhere you're likely to be found when playing music 2) Decide which speakers offer reproduction that most resembles your preconception of how real music ought to sound, using primarily your most naturally-recorded acoustic/vocal source material, played at the most realistic-seeming volumes to suit each program, and positioning yourself in the sweet spot 3) After trying both #1 and #2, if you still can't form an opinion and still like both models equally, then just buy either the less expensive pair or the most attractive pair and be done with it, because you'll know you couldn't have gone far wrong (at least until you come to the point when you've gained some more critical listening experience and may want to upgrade again down the road).
The other important things to stress are that you play the same choice of audition material through both speakers, not just randomly hunt and peck through your collection based on mood, etc., and that you attempt to compensate for any inherent volume differences between the models and then hold it constant between same-program comparisons. I feel that at this time, it would likely be premature and possibly counterproductive to try and give you a laundry list of audiophile attributes or specific audition passages to listen for, rather than just advising you to listen for enjoyment and/or realism as outlined above, or to start talking about optimizing room positioning for each set of speakers individually. With advice from the dealer, just pick one set of L/R spots for your speaker positions that should theoretically work for both models and still be practical for you, and then audition each pair set up in those same spots (you might mark the spots with masking tape on the floor to help switchovers go more quickly). The A/B switch idea is not bad, but actually moving the speakers is better, with one pair 'onstage' and the other 'offstage' at any one time. Try to do both some fairly rapid comparisons, using single musical selections repeatedly, and also some relaxed long-term sessions, maybe on alternating days, just to see if anything crops up that might start to bother you about either of the two models over the long term. Have fun and best of luck!