Well the D50 is
very good for punchy, good-looking, low noise images straight out of the camera - it's well known for that, and in a lot of ways it's a logical step up from a point & shoot camera.
The D70 is less "instant" unless you make a point of changing many of the camera's default image settings, and (to be fair) it has more of a tendency to noise at higher ISO values.
I never bothered with that because I'm a convert to NEF (RAW) photography and can tweak to my heart's content on my PC.
The flipside of the D50's good noise performance is that - due in part to fairly aggressive in-camera noise reduction processing - there is apparently the potential for some loss of detail. To be honest, I think you'd have to be looking
really hard to spot any loss of detail in most pictures that the D50 delivers.
However, some of the settings which literally are at a D70 user's fingertips via buttons or dials on the body of the camera are only accessible via the menu on the D50.
I wouldn't get on with that (I like being able to muck about with various settings, and it's also true that I'm rarely out in the same light conditions twice in a row), but many D50 users don't mind that at all.
In fact, one of my favourite Gallery contributors, Greypoint, was a D70 user for a long time (her images were the reason why I bought a D70) but she's since moved onto a D50 - I believe the D70 needed repair and the D50 was initially a "stop-gap") and in truth, she's never looked back.
But she's using good lenses too...
The D70 uses Compact Flash (CF) cards, whereas the D50 uses SD cards which many people upgrading from point & shoot digitals already have a collection of.
Aside from that, many of the differences between the two cameras are pretty esoteric: no option to have grid lines in the viewfinder of the D50 (very useful for ensuring the horizon is horizontal!); the ISO settings in the D70 are in smaller steps; there's only one command wheel/dial on the D50; the D50 doesn't have a Depth Of Field (DOF) preview button; the D70 only uses an Infra Red remote... and so on.
You should also note that the D70s is different again to the D70 and D50 in several important ways.
None of these might matter to you at all. And bear in mind that the lens you end up with is
at least as important in the equation as the body you pick (and very probably more so, because once you've got your lenses you can change/upgrade bodies, but you'll keep the lenses).
One thing I've read several times about the two cameras is "the D70 is a camera to grow
into, but the D50 is a camera to grow
out of..." and personally I'd say that was about right, but I can't speak for you!
Here's a very useful answer to the same question you're asking:
http://www.nikonians.org/cgi-bin/dcforum/dcboard.cgi?az=read_count&om=15039&forum=DCForumID201
As to lenses: a much tougher question!
In my own experience and for what I do (a lot of "walk-around" photography in local farmland and coastal sites, and I never use a tripod) I doubt very much I could get away with any less than a 400mm lens, though if I had a reserve nearby with hides/feeding stations, I've no doubt 300mm would be fine.
Greypoint (again!) switched me onto the Sigma 135-400mm lens, and I've been very happy with it (though I don't use it now), but her photography circumstances mean that she's getting great images with a quality Nikon
200mm lens, to which she adds a 2x teleconvertor when necessary.
Not using a tripod is a personal choice for me, and I might start using one some day, but this is something else to factor into the lens choice: if you're intending to use a tripod (and there's a
lot to recommend them), you can use longer lenses than I can, even though my hand-holding technique isn't bad, and I use a lens with built-in image stabilisation...
In fact, rather than me rattle off a huge list of
ifs, buts and maybes, tell us a bit more about what kind of photography you want to do - as I suggest, taking pictures of approachable birds at a feeding station and getting pictures of (say) birds of prey at distance, have very different lens requirements - and we'll have another go.
