I run XL in my YLW and I haven't had too many problems with longevity...at least not anymore than I do with any other Medium 'Mech I own. As someone previously stated, Heavy and Assault 'Mechs are the best armor. Don't get in the habbit of making yourself the primary target; when you score a kill the sod should be wondring what the F*&^ just hit him. Myth.
I found a fun loadout with 2 AC2's and a LL in my wang. But I've always had good times with a STD engine too. So I find a Large AC20+STD engine is better than the XL variant. But if you have 2 cannons, you need that XL to make up the weight difference for ammo and energy weapons.
DOES YOUR WANG HAVE STD? That actually sounds funny, huh-huh. But I do agree that making more cbills is a cool feature. That, and the 1x AC20 or 2x UAC5 is not too heavy given the lack of hardpoints on the wang, so STD is the way to go.