No comparison here really, but we can provide the pros and cons as you said d1stortion.
First time I ever played HL was a demo of Opposing Force back in 1999-2000. I remember being scared to death with the headcrab zombies. I played HL in 2001-2002(?) I think.
I didn't play Unreal until 2003, although I was playing a lot of Unreal Tournament and loved that game. Unreal, as much as I like it, I couldn't bring myself to beat it until a couple of years ago when I really set myself down to do it. The thing is, I would always play a couple of hours and then never touch it.
As such, Half-Life for me is the better game, simply because I have played it more and because I absolutely loved the sequel. Both games are revolutionary for their time.
Unreal has a sense of loneliness in a vast and alien world and always tried to show off impressive and moody landscapes, together with some amazing music. The technology was simply amazing, but I always found it iffy. Software rendering was very impressive, but 3Dfx was the only hardware acceleration you could use initially (initial release had absolutely no support for D3D or oGL). And for years to come, the Unreal Engine was plagued with awful performance on D3D hardware. Deus Ex ran like shit on the GeForce 2 for example. Level design wise, some people here said that Unreal had much more vast levels than Half-Life, however very few of them were done right. Most of the time you were traversing an "empty" landscape. My favorite track is Isotoxin, a rather weird but somehow effective composition! The location it initially plays in is filled with enemies that I loved mowing down 😁.
http://www.youtube.com/watch?v=ZiTX4dchAnQ
Half-Life, on the other hand, had a different approach. It felt like every good Action B-Movie should be. Shit goes down, one man mows everything down. The more the merrier they say and it couldn't be any better here, with aliens and marines duking it out and you in the middle of all this madness. People at the time went kinda crazy about the whole concept of persistence in level transitioning and storytelling. I think the cryptic figure that is the G-Man added a lot to this, people going online and noting locations he was sighted in etc... I think it takes a turn for the worse during the Xen levels and it is also one of the best examples of FPS platforming done right. Music was not as memorable as Unreal, but that ending credits music for example was as sweet as anything 😀. Technically, it didn't look anywhere near as good as Unreal, but also had far better HW Acceleration support and had a lighter system "footprint".
Nowadays, playing these games on anything else other than a Voodoo 3 and a fast Pentium II makes us feel as if the games were unplayable, since we are indeed spoiled by 60 frames per seconds and high resolutions. However, I'd say an MMX 233 with a Voodoo 1 was very acceptable for both games at 512x384 or even 640x480 (especially Half-Life). I had a friend who played the shit out of Half-Life on an MMX 166 with Software Rendering.
It's kinda like Crysis 1. I remember being so impressed when I was playing it on my Core 2 Duo 6550 (2.33GHz) and Radeon 4850, almost 5 years ago, and then I was so disappointed when I wasn't getting 60fps on my current hardware a few months ago.