The match will probably be good. Lesnar has had nothing but excellent matches since he defeated Undi' last year and both men will likely want to erase the memories of that subpar outing (which suffered in quality because 'Taker got a concussion in the first few minutes) with a slobberknocker here. They already seem to be benefiting from a superior build: Undertaker's return at Battleground and the brawl the two had the following evening on RAW knocked spots off the clichéd and dull sequence of segments, culminating with a contract signing, the two slumped through before WrestleMania XXX.
There are arguments to be made for both men winning. 'The Dead Man' "needs" his win back and to be rebuilt as a strong force ahead of his presumed retirement match at WrestleMania Play Button. Lesnar needs to remain the most dominant force in the company because that's what people like about him. Obviously only one of them will win. It doesn't really matter who it is. The more interesting topic is what Undertaker is going to do after 'The Hottest Event of the Summer'.
Broadly speaking there are two things he could do. One is disappear again and only return when WrestleMania 32 is the company's focus (which would be any time from January). The other is to stick around and appear regularly until retiring at 'Mania.
I'd prefer the latter. If Undertaker is going to retire next year (and the smart bet is that he will) then he deserves to have one final semi-full time run. We don't need to see him wrestling on TV. In fact we don't even need to see him every week. But appearances on the monthly pay-per-views (or special events as they're styled now) would give 'The Phenom' the chance to work with guys he's never faced before he leaves. Guys like Sheamus, Seth Rollins, Roman Reigns, and Kevin Owens would all benefit from a 'Taker match on their CV. UT would benefit from wrestling these guys as he'd be getting fresh opponents eager to put in the extra work to make the bout memorable, lessening the pressure on him. The PPVs themselves would benefit from 'Taker wrestling as the matches could be pushed, subtly or not, as amongst the last of 'Taker's career.
Is this schedule, or a similar one, likely? We can't and won't know until SummerSlam's bee and gone. We can say that Undertaker looks healthy and put in a spirited performance on the July 20 RAW. But we can also note that he struggled ever so slightly when hoisting Lesnar up for the Tombstone at Battleground. If I had to guess I'd say that we won't be seeing Undertaker wrestling on that many shows between SummerSlam and WrestleMania, a shame because there are plenty of storyline opportunities for him outside of his current programme with 'The Conqueror'.
I'd love to be wrong, and I hope he gets to do something at Survivor Series, if only to allow him to celebrate his twenty-fifth anniversary under the gimmick. That would be fitting if he's on the cusp of retirement.