These are both very good tires. Similar, equally good tires also include the new Bridgestone Dueler H/P Sport AS, the Bridgestone Dueler H/L 422 Ecopia, and the Continental CrossContact LX Sport. The 422 Ecopia and the LX20 both claim to be "ecologically designed" for better fuel economy through reduced tire rolling resistance. All five of these are great all-around choices for an MDX, with a nice balance of price, performance, treadwear, ride, noise, etc. The treadwear warranty is 65K miles for the 422 Ecopia and the 235/65-17 size of the Alenza, 40K miles for the 255/55-18 size of the Alenza, and 70K miles for the LX20. There is no treadwear warranty for the H/P Sport AS or LX Sport.
I've got the Dueler H/L Alenza on my MDX and I like them a lot. Excellent performance (dry roads and wet roads), good ride and noise characteristics, terrific treadlife (the ones on the rear have been on the longest, around 16K miles and only wore 1.5/32" of tread; the ones on the front haven't been on all that long and of course will wear a bit faster than the rear, haven't rotated them yet). From what I've heard (including from the folks at Tire Rack), the others are equally good choices. I don't think you can go wrong with any of them. (You can find side-by-side comparison tests and survey results on the Tire Rack website; overall,
four of these are rated among the top 6 of the 17 tires in their category, with the LX20 and the 422 Ecopia topping the category, while the new Bridgestone is not yet included.)
Personally, I don't worry all that much about treadwear warranties, just because I check my pressures and I've always gotten enough miles that I've never needed to get a treadwear warranty honored. Also the treadwear warranty is usually prorated on a purchase of the exact same tire, so it's not going to do anything for you if you want to replace it with a different tire. And it's not necessarily an indication of how many miles you'll get on the tire; it's quite possible you'll get as many miles on a tire with a 65-70K warranty as you will on one with a 40K warranty or no warranty.
If I were buying new tires for an MDX today, I'd get the 422 Ecopia or the LX20 if I had 18" wheels, and the LX20 or the Alenza if I had 17" wheels.
You may be able to do much better than the prices you mention by buying over the internet and having them mounted and installed locally. Here are current prices via the internet (you won't have to pay sales tax in most states, which may save you almost $20 per tire, but you'll have to pay for mounting and balancing, typically $10-20 per tire; Tire Rack shipping is typically $13-20 per tire).
Bridgestone Dueler H/L Alenza
235/65-17 $137 plus shipping from Tire Rack, $144 with free shipping from Discount Tire Direct, less $70 rebate on a purchase of four through the end of this month
255/55-18 $186 plus shipping from Tire Rack, $199 with free shipping from Discount Tire Direct, less $70 rebate on a purchase of four through the end of this month
Bridgestone Dueler H/L 422 Ecopia
235/65-17 $157 plus shipping from Tire Rack, less $70 rebate on a purchase of four through the end of this month
255/55-18 $186 plus shipping from Tire Rack, less $70 rebate on a purchase of four through the end of this month
Bridgestone Dueler H/P Sport AS
235/65-17 $151 plus shipping from Tire Rack, less $70 rebate on a purchase of four through the end of this month
255/55-18 $183 plus shipping from Tire Rack, less $70 rebate on a purchase of four through the end of this month
CrossContact LX20 with EcoPlus Technology
235/65-17 $145-147 plus shipping from Tire Rack
255/55-18 $188 plus shipping from Tire Rack, $192 with free shipping from Discount Tire Direct
Continental CrossContact LX Sport
235/65-17 $176-177 plus shipping from Tire Rack
255/55-18 $211-215 plus shipping from Tire Rack
HTH