Yeah, the 5000/6000 are a totally different architecture from any earlier Akai sampler. They had a really rough rollout and kind of bombed because of bugs in the early firmware and high price (the s6000 would have been over $4k in 2019 money) but I’ve been using one with the final firmware for a few years and it’s great. No proprietary disc formats or filetypes (everything is windows compatible disk formats with WAV files), audio streaming from disk, live recording to disk from the main outputs and input simultaneously (so in a pinch you can produce a whole track on it, including using outboard effects), 128 voices of polyphony with up to 128 part multitimbrality, 256 megs of sample RAM, 16 direct outs (8 standard on the 5000 but can be upgraded; the 16 is basically a maxed out 5000 with a few hardware extras, but they’re essentially the same machine under the hood). Fully compatible with the open source SCSI to SD adapter that’s available from a bunch of Amiga community shops for decent prices - just pull the floppy drive and mount the SCSI2SD in its place, and jump two points on the IDE pin header to make it think there’s still a floppy drive installed. Can read all proprietary Akai discs and convert them to raw WAV files and s5000 programs, works flawlessly on everything I’ve thrown at it which is more than I can say for any commercial format converter I’ve tried. Two sets of MIDI in/out/thru ports so you get 32 channels of i/o (and it plays standard MIDI files, but I haven’t actually messed with that yet). PS/2 port on the front so you can hook up an old computer keyboard for all of your text entry needs if you want (inside it’s actually a highly custom 386 PC motherboard with proprietary audio, MIDI, disc controller and display hardware, running a proprietary, embedded operating system). Great selection of exotic filter types. The converters are really clean so if you want a character sampler this isn’t it, although they did add downsampling in one of the updates that’s specifically meant to emulate their older machines for people who complained that it was too transparent - by most accounts it doesn’t really nail the sound of the 80s and early 90s classics but it’s still useful for dirtying stuff up. 320x200 LCD display with 16 context-dependent buttons so it’s easy to jump around to different pages and parameters - it’s pretty close to self-documenting for the most part. The data encoder can get a little fidgety on them but it’s actually designed in a way that you can actually disassemble the encoder, clean and lubricate it and reassemble it without even opening the case, which is a nice unintended feature - mine showed up with a jittery encoder and it took 5 minutes to clean it out and make it work flawlessly with nothing except a pair of needle nose pliers, a couple Q tips and some deoxit and lubricant; there’s a good video somewhere on Youtube that shows how to do it and it’s less than two minutes long.
The main thing I’d say is only get an s5000 if it has all of the expansions you expec to need installed already, because they’re rare and expensive to get on their own. Especially the effects board, those are usually around $300 these days and at that point you could just buy an S6000. The effects aren’t supposed to be amazing but they’d be nice to have and I kind of regret not having them (but with 16 outputs you can just use the Octatrack or other outboard effects - you can even sample from the inputs during playback so you can sample with external effects, which isn’t anything special in Octatrack world but was pretty advanced for a sampler in 1998.
It doesn’t do anything exotic (other than some of the weirder filter types) but it’s a bread-and-butter workhorse with a really fast, intuitive workflow, and it doesn’t have the “vintage mojo” yet so the prices have risen but the price/performance is still really good even by modern standards IMO.
Oh, don’t bother looking for the USB addon - it’s the rarest and most expensive upgrade, it’s USB 1.0 so file transfer is impossibly slow, and by all accounts the proprietary control software they released was buggy at the time and is almost impossible to run on a modern operating system; I briefly considered getting one at first but it sounds like a huge waste of money).
The Akai z8 is even more advanced (it was the last generation) but the UI looks like a big step down to me and I’ve never used one so I can’t vouch for it. Could be worth considering, though.