Thanks everyone for all the great responses.
CD, you make the point a clutch can be set up for a fine degree of control. Not knowing much about sewing machine motors, I had always heard of feathering in the context of cars and trucks and I've always been told that partially engaging the clutch (say, on a steep incline from a stopped position) is hard on it. Greasing the cork is an interesting idea but I wonder if doing so provides slippage at higher speeds when control over small increases/decreases in speed aren't such an issue -- or does pressure overcome this?
And your point that refurbishing a vintage industrial to original specs (or as original as possible) means a clutch motor. I've seen some beautiful clutch/table sets in recent months and some of them can really haul.
I suppose I had already answered my own question and was looking for confirmation or some input that might get me to change my mind, but I think I'm headed in the servo direction. The machine's a modern machine, so no vintage considerations. It will be used inside a home and servo motors are a lot quieter and draw far less power. And they're easier to move.
miriam brings up cost. The cost of the current machine/motor/table was so low that buying a new servo means I'm still well within a low budget. And from CL, eBay, etc. listings, I gather that servo motors (except for those like CD who want a completely vintage set up) are far more desired and thus more valuable and easier to sell. If and when I decide to sell this machine, a servo will make it far easier to do so. Prices for servo motors are around $150, some more and some less.
mtpockets brings up ease of installation and that's yet another advantage (not the least of which is weight!). I won't be using the machine daily but a speed reducer and/or a pulley that places more torque at the needle would likely be to my advantage, as I'm not in need of 5500 spm at any point and would like greater flexibility of a light- to medium-duty machine rather than one that goes incredibly fast but is mainly for lightweight application only.
I believe that as a relative beginner, I would be better 'served' by a servo. The Mitsu clutch on my Adler 98 is hard to control and the machine itself is burly and could sew through my arm. Kind of intimidating.
I won't be getting the servo right away (I'd like to see what's wrong with the clutch and try to fix it if I can) but I'm definitely leaning in that direction.