I've not used a 512mb WP, only 1 gig. The only android I can compare my 640XL to, is my Note 3, which had MUCH more ram. Therein lies the problem. Android is a resource hog, and memory bleed is a nightmare. I had issues running some basic tasks on the Note because the Android system keeps apps open in the background, only dumping them when you run short of ram! Most droider's will have some form of memory manager, usually C Cleaner or Cleanmaster.
The thing I noticed with WP is the way in which the ram is managed. I've not seen a glitch on the XL. My son has a lowly 435, and even that comes with 1 gig ram, and that plays games very well indeed, especially when you add in that it only has a dual core processor! The hardware seems to be better managed all round. So IMHO, you don't need to suffer 512mb, unless it's a 635, and with the pricing of WP, why not spend an extra few quid for the 640 or save a shed load of cash and buy a 435 for ?30!! I've now got one as a spare for that kind of money! Yes it's only 3G, but as a back up, it's more than capable!! We've also got a 535 (broken screen, arrrgghhhh) that for ?50, also runs 1 gig, and is more than capable, although also only 3G....