Someone over at rpg.net asked about the differences between the two, so I responded based on my reading of the PDF so far. I then realized "I should put this on the blog," so here goes:
Magic Worlds vs. Runequest 6....initial similarities, differences
Character options: MW is BRP based so it doesn't have a culture distinction, but breaks down by professions in a BRP format. It's BRP-compatible. (EDIT: Yes it does. I skipped over it on the initial breze through).
Health/Hit Points: It has some rules on more heroic level HPs as an option (but not default). RQ6 has a culture distinction and breaks hit points down by a location chart (BRP offers hit locations as an option, but MW defaults to a single HP value...which I personally prefer).
Resistances/Saves: MW uses the resistance table and the BRP concept of characteristic rolls (like BRP). RQ6 uses two skills, endurance and willpower (formerly resillience and persistence in Legend) to handle many "save" effects.
Magic: RQ6 has several magic systems of different flavors, which you can use all at once or individually to different effect. MW uses one magic system (sorcery) derived from Elric, so it's a more heavy-hitting swords & sorcery feel set of rules with a lot of demonic and elemental summoning features. The MW sorcery is about as expansive in terms of options as the combined folk magic, sorcery and animism rules in RQ6. MW does not have anything to compare to the mysticism and theism of RQ6, although it is compatible with BRP Magic, which is a recreation of the RQ3 magic rules.
Combat: RQ6 combat is maneuver based, and MW is as well, albeit in a slightly different format. If I had to call out one or the other as more detailed/complicated I would say RQ6 is the more complex, but really we're talking apples and oranges here....the main difference as stated earlier is that MW does not use a hit location system to break down HP like RQ6 does, relying instead on a core HP total ala BRP.
Monsters: Both games have robust monster selections, with some variants between the two. Offhand the RQ6 monsters has about 60 entries and it looks like MW has about 80. Both provide a good range of support for different conventional monsters types, with the usual quirks of the RQ/BRP family. MW is more conventional, however (i.e. earth elementals are not named gnomes) with a bigger array of familiar beasts but fewer uniques. Both games snub goblins for some reason (goblins are missing from MW and RQ6 lumps them in with orcs, which becomes a broad category for "humanoids we fight.) Don't know what the lack of goblin love is in either game.
GM/Campaign Stuff: MW has some detailed ship/sailing rules in it (RQ6 doesn't). RQ6 has specific rules on cults and brotherhoods (an important part of advancement and interaction in RQ6) but MW does not use this structure as such. Both games have some nice GM advice sections, with MW being geared toward somewhat more traditional swords & sorcery (by a slight margin) while RQ6 hangs closely on its "mythic ancient world" style. MW has a default core setting it includes in back as an example starting point. RQ6 plans to add setting in future books.
Magic Items: MW has a big plus in that it includes an array of interesting magic items to include in a campaign, which is cool but not derivative of conventional D&D-like approaches to magical devices as such. RQ6 has/will have enchantment rules, but magic in the form of items is considerably less normal in a typical RQ campaign.
Artwork: both games have great artwork. YMMV but I think I lean slightly toward MW's art style and layout, which has few "white spaces" than RQ6 and the font is easier on my eyes.
Both games are all-in-one packages for the most part, so with just these core books you could run a full campaign in either one. I might give a +1 point to this on MW since it also adds the ship/sailing rules, and offers a default setting that a time-restrained GM can immediately get into. MW is also more focused on a specific style of sword & sorcery play, so it strikes me as easier to digest the single but more robust sorcery mechanics, where as with RQ6 you need to learn five different magic systems.