Having owned 2 Abolts, one Xbolt and numerous Vanguard and Howa rifles here are the major differences between the 2.
1. Drop forged receiver, simple bolt design that is completely field strippable.
Actually the Howa/Vanguard receiver is extruded. You can take the firing pin assembly out without tools on the Howa, while with the Browning you'd need a pair of pliers, vice-grips or similar (or a vice) to pull the cocking piece back. For stripping beyond that point both require tools - and much the same tools.
FWIW In several decades of hunting I've never had to strip a bolt, on any rifle, in the field, so the difference is moot. YMMV.
2. X-bolts receiver bridge is so thin it requires the use of 4 screws on each scope base.
4 screws would be stronger than two too.
The x-bolt has 2 advantages low bolt lift and lighter weight. Old and New vanguards are some of the most consistently accurate rifles one can buy with exceptional quality control.Too much cheap metal in the Abolt/Xbolt trigger and bottom metal.
As far as I've seen, both are very accurate, and have very good QC, something the Japanese are very good at, owing, among other things, to work done post war by Americans like W Edwards Deming.
As for the "cheap metal", it seems to me that quite a number of makers use aluminium alloy for bottom metal, not just Browning, and not for cost but lightness.
In the trigger mechanism the working parts are hard-chromed steel. 
