I've own a Viewsonic 32" N3250W 1080i. and a Toshiba REGZA 42" 1080p 42RV530U.
For my Viewsonic 32", it's such a small screen, can't tell the different between 720p to 1080i, because the screen so small, I've read some facts about HD TVs, lower than 50", you only need 720p. Sometimes it's not because you need it or not. It is something has to run in 1080i or p but not in 720p, some only run in 720p but not 1080i or p, so depend what the media needs.
For my Toshiba 42", I don't compare to 720p to 1080p, I compare 1080i to 1080p. It has a slightly different. It doesn't have different even lower than 50" even tho some pro said it doesn't, but IT DOES.
Also, another fact is, my 32" only runs 1366x768 resolution even it's a 1080i HDTV.
For my 42", it could run in best 1080p resolution and have the benefit of the full HD for computer, which I'm very happy about this.
OK, for those who use their HDTV more like a monitor than a TV, you probably want to look at the resolution more than the HD value, but of course, 1080p is good.
For me, I use my 42" in my room, also use only for computer, and I do some design work like in Photoshop, so, larger resolution help me a bunch. I also use it for PS3, it looks perfect.
Another thing is the features. If you prefer more features, you would have to go thru the system. One thing is very sad that my 42" doesn't have PIP or POP feature for such a new model TV, but I hardly watch TV which is OK to me, but it's a shame not to have that feature.
For my 32", it has a very big and superb speakers which I don't have on my 42". The bass is very good even it's on max that you can't hear any "Paper Flapping Sound" (don't know how to describe it, you know, cheap speakers has that). But on my 42", it has a line speaker which is not a big speaker very obviously, delivers a very bad bass sound.
I chose Toshiba because it's a nice durable brand. I don't mind the speakers because I have a surround sound system so I don't need it unless I use it at midnight.
Problem is, my 32" broke after 2 years on the power supply, so I bought 4 years warranty for my 42". Now I have experience, I don't buy it from here, especially with less feature, not enough warranty, refurbished, and the way how the deliver man treat the packages.
Such a long story, so guys, choose the TV wisely.
Viewsonic N3250W 32" (See the speakers are kind of big which deliver a superb bass sound)
Toshiba REGZA 42" (See there is only a small place for speakers, sadly it can use medium bass only)
Some more related info about HDTVs.