Having had a TiVo for about a year and a ReplayTV for about 3 weeks (and just received my second last week), I'm still trying to decide for myself which I like better.
Both offer similar overall functionality, but both have strengths and weaknesses. I only have one TiVo so I haven't been able to play around with the "multi-unit" features, but my impression is that RTV is much better in this area. But I really do like the MP3 playback feature of my TiVo since it's a perfect way to tie my PC-based MP3s into my Home Entertainment System.
I haven't hacked around with the TiVo much yet, but I think RTV is also better for power users that aren't afraid of PCs, routers, and playing around with tools available on the Internet.
Given the great deals on the 5040's, I'm actually considering getting a third unit and then upgrading the hard drive, but I'm trying to resist the temptation since I already bought 2 last month.
Once setup, I think anyone could be satisfied with either one, but for the "technically challenged" like my mother, I'd probably recommend the TiVo and for the "technically savvy" I'd recommend the RTV.