I own and (have) used both.
When I had a 9' long reloading bench, I had both set up (and others). Now that I have a 4.5' bench, I just have two presses set up--the Loadmaster and an updated standard Turret.
The Load-Master has two major advantages over the Pro 1000 in its design and features.
1. It can do larger / longer (rifle) calibres.
2. Its five-station turret allows for die configuration flexibility.
And it much more 'heavy duty.'
Were I just wanting to build some semi-auto fodder--good reloads, to a custom recipe, but not carefully-finished (so to speak), I would re-install the Pro 1000. If that is what you need to do, then the Pro 1000 is a good choice.
I used mine to build 1000s of .45 ACP and 10 mm rounds fifteen years ago. These were non-max loads that provided barely-major PFs, built to the accuracy-load sweet spot in the respective pistols (different 1911s, and others), and once the combined seater-crimper die was properly tweaked, 400 rounds an hour could be sustained in production. I set up turrets and complete carrier assemblies for the respective calibres, and I could change out calibres in about one minute.
This year, when I set up my bench again (new house, eight years away), I was working with different calibres, and I wanted the flexibility of 4-die configurations--i.e., separate seating and crimping. (I was embarking on a reloading project with revolver ammo that would provide separate seating and crimping, and I was working with max loads.) I wanted the ability to tweak the finish of those rounds carefully once I did production runs--so I set up the Load-Master (and I updated one of the standard Turrets to 4-die configuration and added the Safety Prime and installed it as well).
All Lee presses have their "quirks"--and a successful setup on the Load-Master is arguably more difficult than one done on the Pro 1000. Quite frankly, I had been wanting to throw mine away when I last used it--but I took one last shot at using it by starting with a detail-strip-and-clean, studying the support videos, using that turret flexibility to provide for separate decapping and sizing, and then finishing with careful die setups.
It paid off, and the Load-Master is working smoothly--because of its construction details, it can run easier and faster than the Pro 1000.
Once done, however, the production is higher. And, more importantly, greater flexibility is there. The higher production rate of the Load-Master is no longer important to me, but that die configuration flexibility is.
EDIT: FIEROCDSP types faster, so I'll add on some reactions--
1. about spent-primer cleaning--FieroCDSP, use a bent coathanger to clean out the ram. It's bent to get your hand out of the way for the 'plunging' one needs to do--i.e., from the bottom. That is easier than disassembly, IMO--and I've even run Gun Scrubber down the ram and swabbed it successfully with the press assembled.
2. The indexing difference in the two presses IS important, and better in the Load-Master--the Pro 1000 indexing parts I keep on hand, because they can be damaged and if they are, it's a no-go until they're replaced.
3. The Z-bar on a Pro1000 can be problematic, but some straightforward Z-bar tweaking generally fixes it. What happens is that the Z-bar tab for press connection also gets worn, and people don't notice that. The Load-Master feeder setup is more complex, IMO--but works as well or better if one faithfully uses the YouTube videos--and uses the STP lubrication trick.
4. That comment about press user types is important; pay heed to what kind of user you are.
Jim H.