This may surprise some of you but I actually see this being quite close. And I can say with 99% certainty the much-celebrated '99 final was NOT Pete's best match at Wimbledon. How come? 'Cause his return game in '99 wasn't as good as it used to be.
But first let's look at his %s of service games won in his championship runs:
1993 - 93.5%
1994 - 97.2%
1995 - 93.9%
1997 - 98.3% (this is the year when he held serve a mind-boggling 116 out of 118 times)
1998 - 95.0%
1999 - 95.0%
2000 - 95.9%
Average - 95.5%
You can see his hold % held steady (haha, geddit) throughout his prime. But it's a different story with his return %s:
1993 - 21.7%
1994 - 26.5%
1995 - 25.2%
1997 - 26.5%
1998 - 20.5%
1999 - 24.0%
2000 - 21.3%
Average - 23.6%
Now 24% on low-skidding grass is still pretty damn good, but compared to his peaks in '94, '95 and '97 ('95 is somewhat marred by his 5-setter with Goran who served the lights out in the SF) it's still something of a disappointment. I've long pointed to his '95 final and '97 QF against Becker as two of his very greatest performances, and I'd feel safe betting on his '94 and '97 iterations (he struggled with his 1st-serve % throughout '95 though not so much in the final vs. Boris) beating his slower version in '99 more often than not.
Alas we don't have a breakdown of Borg's performance in the service and return departments from '76-'80 (if there's a Borg fanatic out there who does have those #s from his individual matches, do let me know), but one thing we can compare is the overall % of games won by each player at his respective Wimbledon edition. So let's look at Pete's #s 1st (I'm including TBs this time around):
1993 - 58.8%
1994 - 63.2%
1995 - 59.5%
1997 - 62.7%
1998 - 58.2%
1999 - 60.4%
2000 - 58.8%
Average - 60.1%
So Pistol won over 60% or close to it and averaged 60.1% in his championship runs. GOAT stuff for sure, but what about Borg? Turns out he's also up there:
1976 - 65.5%
1977 - 58.5%
1978 - 61.3%
1979 - 59.8%
1980 - 61.4%
Average - 61.1%
As you can see Bjorn actually did better than Pete, this despite the former King of Clay's well-known difficulty adjusting to grass in the earlier rounds. Maybe this matchup wouldn't be so easy for Sampras as most of you seem to be thinking?
Ah, but we all know Pistol raised his game for the finals, right? Here we can make a more in-depth comparison as stats for all of these two's Wimby finals are readily available. Here are each player's %s of overall (again including TBs), service and return games won in their finals (afraid our statheads neglect the SW/RW% part, so I had to get Borg's #s from TA's point-by-point descriptions)*:
-Sampras-
1993 - 52%, 90%, 10%
1994 - 63%, 100%, 20%
1995 - 62%, 100%, 26%
1997 - 64%, 100%, 29%
1998 - 53%, 92%, 15%
1999 - 61%, 100%, 20%
2000 - 57%, 100%, 14%
Career - 57.9%, 96.9%(!), 18.3%
-Borg-
1976 - 62%, 88%, 35%
1977 - 57%, 74%, 39%
1978 - 72%, 92%, 54%
1979 - 56%, 96%, 17%
1980 - 51%, 89%, 15%
Career - 57.7%, 87.4%, 29.1%
*Strictly speaking we do know which TBs were service games for either player (FYI Mac served 1st in both Borg's TBs so they were return games for the Swede), but for simplicity's sake I'm counting them only in the overall GW%s.
Pretty damn even. I think we can safely assume that even after factoring in the extra boost Borg would get in SGW% from today's gear there's no way he's touching Pistol's mind-boggling 96.9% - yes, the guy held serve more often in the finals than in the previous rounds, which I highly doubt any other multi-Slam champ has ever pulled off at any of the four majors - while Pete in Bjorn's place probably doesn't average 29.1% on return. And though it's easy to say Borg had to deal with not one but two ATGs Courier in '93 was in the proverbial zone, having destroyed Martin's and Edberg's serve in the previous rounds - not to mention that a great serve alone can carry you far on grass, as we can see from those low RGW%s in '79 (vs. Tanner), '80 (Mac, of course) and '94/'98 (Goran). And guess who's got arguably the most lethal service game of 'em all?
Of course this matchup wouldn't be taking place on paper, and while it's hard to bet against Sampras at Wimbledon remember what they used to say about Borg not being a natural grass-courter vs. what actually did happen. And one more thing. In the deciding set of the '80 final
Borg got 74% of 1st serves in... and didn't allow Mac a single one of the 13 BPs he earned in the 1st four sets (hat tip to krosero). That's why none other than our resident statistician Moose has described it as perhaps the most clutch serving he's ever seen in the 5th set of a major final (or something to that effect), and while you could quibble with the admittedly hyperbolic assessment you'd be hard-pressed to name a better example.
Long story short I do think this would be very close. If Pete can't close it out in 4 I can see Borg taking it in 5, and over the course of a long series the two might well end up splitting it 50-50. If we were talking about '94, '95 or '97 Pistol I'd be backing him without much hesitation, but not this one. '80 Borg does not lose in 3 to anyone, on any surface and certainly not at the tournament he won five times in a row.