... I've had this thing serviced a bunch. Wonder whether Epson V800 series (850) with Betterscanning's frame holders is a better current-day replacement? ...
Wonder what's your experience?
The Nikon will almost certainly give you higher actual resolution and better dynamic range. The dynamic range I believe will mainly mean better shadow detail. However it all depends on what quality level you want. It's a trade off, money for quality. For example, you could probably get even better quality than the Nikon if you bought a drum scanner, but the cost and inconvenience might make this a bad idea. The Epson might be good enough for your purposes, particularly for medium format. Advice here is good, but the only way you will know for sure is if you buy (or borrow) one and compare.
Here's my suggestion, with an eye toward conserving money. For 35mm buy a used Canon FS4000US on ebay for about $150. (Be sure it includes the film and slide holders because these are no longer available from canon.) This will give you scan quality for 35mm that rivals that of your Nikon. Buy Vuescan professional edition for $89 to run the FS4000US. (It will also run almost any other scanner.) Buy an Epson V800 for medium format and large format. If you find that golden negative or slide that absolutely needs higher quality than the Epson can supply then have that negative or slide scanned by someone who can do a drum scan at about $100 per photo.
The Epson V850 may or may not be better for you than the V800. It has a few extras, but they may or may not be worth it to you. For example, I believe the V850 includes the silverfast SE Plus scanning software, but chances are you will do just fine using either the software supplied with the V800 or vuescan for scanning. The V850 also includes some extra film holders.
This scheme is probably a 90-95% solution for half the money compared to buying another Nikon scanner.