All 3 series are basically the same the only differences is that the 4xxx and 5xxx have automatic commercial advance, which is awesome. Also 4xxx and 5xxx are slightly louder than the 3xxx because the 4xxx and 5xxx have case fans. Oh yea, the 4xxx and 5xxx are broad band capable so you can download the channel guide via the web and there are a bunch or programs out there that will let you connect to your replay and download shows to your pc, there's even one...revue I think...that will let you stream shows to your pc, so some could be watching tv with the replay and you can watch any saved show via your pc at the same time.
If your going to buy one I'd say definitely get a 4xxx or 5xxx series because the ethernet port is a nice upgrade over the 3xxx. Also if your planning on upgrading your replay with a new hard drive go with a 4xxx or 5xxx because the case fan really helps to cool the unit. The old 3xxx units used a 5200rpm hard disk so if you replace that disk with a 7200 rpm disk the replay can get pretty hot.
Either way I'm sure you'll be happy with any of the replay's they are great units. I've bought 3 replays in the last 3 years, I bought a 2020 that I upgraded to 80hours, then I got a 4040, and finally I bought a 5040 that I upgraded to 125hours.