My high run is higher on a 9 footer than an 8 footer, but of course I do play on 9 footers almost exclusively nowadays. I've broken the 100 mark on both. My 8 foot record is lower than on the 9. I haven't run those kinds of numbers in some time. Back is acting up and have taken many breaks from the game in the last few years for that reason. I've also not played as much straight pool when I have been playing. I'm back at it now, though.
It's hard to make good comparisons, because the 8 footer I've played the most on was more like a home table, slow rails and big pockets, while the 9 footers I play on are tight, double shimmed, with bouncy cushions.
Most likely the 8 is easier, based on length of shots etc. There is more congestion, but hard to get into really big trouble on the 8. Now if it was a REALLY tight 8 foot table (which is hard to come by), with really bouncy rails, like a tightened Diamond, then I'm not sure. Might be tough to negotiate traffic then, and hard to shoot the inevitable combinations.
If you are interested in the musings of a pretty average pool player, like myself (at least as far as avid straight poolers), here is more: I find that I like to finesse the rack more on an 8 footer. Sometimes you get punished if you go to hard into the rack, by having balls group up on the rails. On the 9 footer I like to slam the rack more, since it tends to pay off more. It also seems a lot easier to get good breakshots on the 8 footer and key balls seemed less critical. Going up and down the table was no biggie and ending up with a long breakshot was not the end of the world. Now, if it was extremely tight, like 4 inch pockets, then that might compensate and make it almost as tough. With any kind of normal pocket dimensions, I do think 8 footers are easier. If you play a lot on the 8 footer, I'd suggest perfecting under the rack breakshots, and breakshots where you go into the top of the pack or side breakshots played with force draw. Often the perfect Mosconi breakshot can give you trouble (on fast cloth). On the table I played, the "perfect" breakshot would often cause 3 or 4 balls to be pulled down onto the bottom rail. Not good. That was compounded by the rails being slow, I think, which is probably typical for most 8 foot home tables. There is not a whole lot of room to maneuver under the rack after the first breakshot on an 8 footer, and very easy to get doubled up down there. You need the cueball at center table. Side breakshots with follow can sometimes end up below the rack because of kisses and just because the cueball expends its energy. Those are my findings.