Urine will pit aluminum floor's if they're not maintained.  Like pulling mat's out every 3-4 month's, washing the floor and mat's and putting the mat's back in. 

Now, that's not my idea of fun.  Those mat's must weigh a ton and they stink!  Even after washing.  When ordering our trailer, the dealer offered WERM flooring.  After looking at a trailer that had it, we were all over it.  It added $1000 to the price of a 4 horse.  Well worth it.  And it is almost too good to be true. 

Just think, when cleaning out your trailer, sweep it as clean as possible.  Take it down to the car wash and wash it out.  Cost about $5.  Clean fresh smelling trailer. 

 Regarding slant load v straight.  I had a straight and horse was always sweated up when we arrived.  Not so in my slant load - travels more happily.

As for rear tack, some trailers (i.e. Titan) come with a collapsible back tack area and removable saddle rack, so you can have it in the back or in the dressing room.  You can load all saddles, etc. in dressing room and then when you get to a show, campsite, etc., you can re-establish the back tack area.  Very easy to do.

Good Luck!