With only two or three compaines making 1:56 scale plastic kits, after reading this someone might wonder why don't these two to three companies want to work together instead of against each other? Because it seems like a waste of resources when we get almost the same plastic kit from two or three different companies.
My sense is that Warlord have deliberately undercut Rubicon in several cases. I can't prove it, but their releases have tended to track Rubicon's (Rubicon appear to have a longer lead time from announcement to release). I bet Warlord see Rubicon as pretenders in a market they'd pretty much created (plastic 1/56 scale vehicles).
A are a few examples:
8.8cm Flak Gun (RM & Italeri) - both more than good enough for the gaming table. (1)
Jadgpanzer 38(t) (RM & Italeri) - both more than good enough for the gaming table. (2)
Panther Tank (RM & Italeri) - both more than good enough for the gaming table.
M8/M20 Armored Car - both more than good enough for the gaming table.
Tiger I (RM & Italeri) - both more than good enough for the gaming table. (3)
Stug III (RM & Italeri) - both more than good enough for the gaming table. (4)
M10 TD (RM & Italeri) - both more than good enough for the gaming table. (5)
M3 Halfrack (RM & Warlord*) - both more than good enough for the gaming table.
SdKfz 251 Halfrack (RM & Warlord*) - both more than good enough for the gaming table.
Opel Blitz/Maultier (RM & Warlord*) - both more than good enough for the gaming table. (6)
* There is another company that makes plastic kits for Warlord Games, but I'm not 100% sure who it is. Plastic Soldier Company?
I would caveat some of those comments. You're correct about them all being servicable wargaming models, but there are still significant differences in quality and detail.
Warlord/Italeri's 88 doesn't include plastic crew, and they didn't do the tractor in plastic.
I think Warlord's Panther is noticeably better, provided you like Zimmerit. However, the tracks are a bit annoying.
Rubicon's M8/M20 is, I think, more crisply detailed and with complete wheels.
Warlord/Italeri's late Tiger I is better than Rubicon's, but Rubicon's kit makes a nice early Tiger (with a bit of work).
Rubicon's StuG III is, unfortunately, rather inaccurate. Warlord/Italeri's is greatly superior - apart from the tracks and the limited options.
I thought the detail on Warlord/Italeri's M10 was rather soft, and the crew were pretty horrible. While Rubcon's M10/M36 has a few issues, I think it's much better (and you get the M36, which is rather good from a wargaming perspective).
I think Rubicon's SdKfz 251 and M3 halftracks are vastly superior to Warlord's.
I don't have the Warlord Opel kit - it looks like better value in terms of options, which is unusual.
There are also the Sherman kits, which partly overlap (Rubicon hasn't yet done an M4A4 and Warlord only has that and the M4). Here, I think the Rubicon kits are also vastly superior, not just in terms of detail and ease of assembly, but also the range of options. This is where competition has helped us, because Rubicon have looked at what Warlord did and improved upon it.
PSC appears to be supplying designs for some kits - you can tell from the scaled-up figures (identical to PSC's), component layout and oversize details.