The HP/Agilent 34401A is a classic and came out in the early nineties. A lot of information is available online, including service documentation. It is a solid performer without major flaws, which explains why it was for sale from the early nineties until the introduction of the 3446x series a couple of years ago.
The Fluke 8845A/6A are much more modern, were introduced about ten years ago. Has more features: current ranges, trend plotting, modern connectivity (LAN). One thing to watch out for is that the 2x4 Ohms input connectors can become flaky. There is little to no service documentation available (the user manual refers to a "service guide", but I could find no trace of this document). You should be able to find robrenz's musings about how great the Fluke 8846A is on this forum

. The 8845A is very similar to the 8846A (I believe the 8846A adds temperature, capacitance and USB host).
If both are working, I would definitely prefer the 8845A to the 34401A for bench use. For systems use I might prefer the 34401A because of reliability and wide support. I would much rather repair a broken 34401A than a broken 8845A, however.