Hi Jason,
First, I agree - it's near impossible to find a Yorkville dealer with any higher end boxes in stock for demonstration(i.e. Elites, Unity's and TX's). It' a shame actually, because they'd sell a lot more of 'em. I digressed however.
The EF500P is, in my opinion, one of the better self-powered 2-way 15's available, especially when you consider it's price point of under $ 1k (shop around and you'll find it for less than $ 950). It's an in your face, no excuses speaker that never fails to impress. A perfect mate to the LS800P or LS808.
The U15 on the other hand is slightly more refined in it's approach/sound - providing more mid-range warmth and slighty more high frequency extension. Think of it as a studio monitor on steroids (ok lots of steriods). Depending on material & venue however, you might opt for 2 subs per U15. Don't forget it's got (3) midrange drivers and one HF CD - that's a lot for one 15" to keep up with.
You didn't mention enough information however for me (or anyone else) to make an informed recommendation. Will you be playing indoors? outdoors? crowd size??? Do you plan for one per side or more than one per side? Do you have any other requirements that might help dictate product selection? I.E. if you already have an investment in amps, the U15 might seems more desireable, but if you have no amps and starting from scratch with a new system, do self-powered speakers appeal to you?
Best wishes,
ScottH