I'd say definitely the artifact, unless you've got the basic resistances elsewhere (it provides all the basic, light and dark and probably nether) - you don't get hit by that many missiles, and they're generally not very powerful until quite late on; you'll be perfectly comfortable taking on knight archers, quiver slots, novice rangers and black orcs without reflection. View reflection as a nice bonus, but don't drop resistances or other abilities ('cept maybe feather falling) for it. The artifact will also resist disenchantment better than the other one, and not be damaged by elemental damage. You could always keep the reflecting one in a home in case you need to fight a bunch of tough missile firing monsters later.
The flamberge will do more base damage on average, but the lance has a slightly better chance to hit, is possibly lighter (so you'll be less burdened), and has the fire brand - it does double damage against monsters which don't resist fire, making it better overall. It'll provide fire resistance too. Worth trying to get that +dam on it up with a few scrolls of enchant weapon deadliness; you'll probably start finding them before too long, but they're pretty affordable in the town. If you're a chaos warrior, don't bother - odds are your patron will give you an ego weapon or chaos blade before too long anyhow (unless it's Balo).
Regarding quests, if you've not done any at all yet, then start with the first one given by the mayor in the outpost - it's entirely possible (though not adviseable heh) to do that one right off the bat before you go into the dungeon for some characters. You get an enchanted longsword. The sewer quest given in the expensive town whose name I forget (east and north of the outpost) is dead easy, just be prepared to kill the gremlin before it can breed and have a source of see invisible or detect creatures. There's a ring of light and darkness resistance as a reward. Then go back to the mayor at the outpost and do the warg quest on level 5 of the dungeon for plenty of experience and a shield to sell. After that, the next one you'll be able to do is the orc camp quest, but that's going to be tough for a while yet, especially if you've not done it before and don't know what you'll be facing

For that one, get your stealth up if you can, it's much easier if you can avoid waking all the orcs up.
There's nothing stopping you accepting all the quests you're offered, btw - there's no time limit, and the descriptions include an estimate of their difficulty (expressed as an equivalent dungeon level) - unless you've done them before, add about 5 to that number before thinking about them - some are quite heavily underestimated (others overestimated, like the bandit camp).