As someone who lives out here, here are my two cents.

AUH airport is far easier to get through immigration and also security. The BA flight arrives at a good time for immigration, with most people on the BA flight using the automatic passport barriers - you'll be through within 5 minutes tops. Same on your departure, if you use the Business Class/First Class dedicated security lane, then you'll be through security in less than 5 minutes as well. There is also much less walking than having to schlep to Terminal D at DXB. The gate enforces a very strict F/J class boarding policy before all the hordes.

Now, the lounges. I don't use the normal terminal at AUH - I use the VIP Terminal thanks to my Mastercard Platinum (if you have one, you can use it too) however, in the past when I did use the normal lounges, they were fine, but yes, quite busy. However, if you think the BA DXB lounge is any quieter, then you're wrong. It's always full to the gunnels. And, the CCR Bar there is crap. It's always full as well. The way I would describe the BA DXB lounge is a bucket and spade lounge for those with money (full of kids and self pompous parents).

787 is far better in J/F than the 777 or 747.
787 is also far better in W than the 777/747.

P.S. Good luck driving here - get used to people not using their indicators, no real lane management, and tailgaters - It takes a bit of getting used too!