Here are the results for all the online events in which both players have competed:
Event Brian Allen Allen Staal
Wildcard Rematch #2 350,400 235,800
Wildcard Rematch #3 / Wildcard Qualifier #1 637,800 311,600
Wildcard Qualifier #2 412,000 295,700
Wildcard Qualifier #3 341,000 567,200
Donkey Kong Online Open 2014 #1 474,100 556,300
Donkey Kong Online Open 2014 #3 447,700 386,100
Donkey Kong Online Open 2014 #4 522,600 603,200
Donkey Kong Online Open 2014 #5 653,700 450,500
Donkey Kong Online Open 2014 #6 464,600 753,000
Donkey Kong Online Open 2015 #2 868,200 625,900
Donkey Kong Online Open 2015 #3 845,400 808,600
Win Total 7 4
Average Score 547,045 508,536
Brutally honest analysis of both players:
Not a huge difference in average score, and a large variance for both players. Both players have been "hot" in recent events, but neither of them are consistent high-scorers. Brian has only been save-stating L4 barrel board bottom hammers for some reason and unless he's been practicing offline there's no reason to think he's capable of taking a game deep at anything higher than 875k pace. Allen is lucky to maintain an 855k pace, although he did have a 600k game at 888k pace recently. Both players are easily flustered and dismissive when the competition gets heated. Allen values TG recognition much more than Brian (as evidenced by his attempts to create all the shitty TG tracks), but Brian values his overall reputation as a skilled gamer more than Allen. Brian will hate losing more than Allen will enjoy winning (Brian hates most things anyway).
It was suggested recently that if Brian puts up 700k on the the first day that he'll essentially have the competition locked up, as this would immediately put intense pressure on Allen. However, a 700k score would be his 3rd highest score ever. In fact, I believe Allen has more games over 600k than Brian does. Brian, unsurprisingly, has complained about the start time of the competition, but with a 10 day competition there is more than enough time for both players.
Ultimately, the stakes are higher for Brian than they are for Allen. This gives him an incentive to come up with an excuse to bail out (not unheard of). Brian is the better overall player and has a winning record against Allen. Allen, with his notorious lack of fundamental DK skills, will need to luck into a big first man in order to get into the zone he needs to go deep. Even then, Allen might only score 860k.
My prediction: BA wins, but JCB beats them both.