My experience has shown me that the Yamaha is stronger as a one-man-band unit right out of the box, but the Korg is more of a player's keyboard, and has a more professional edge to it.
Some important areas of comparison:

construction: Korg wins hands down

key feel: Korg again

sounds: pretty even across the board

styles: Korg is more exciting, Yamaha is more generic. That's a plus if you play dance music, and want simple, basic dance beats. The Korg drum tones are FAR better though.

fills: Korg overdid alot of them - Yamaha got it right.

harmonizer: Korg doesn't give you presets like the Yammy, but it DOES have great sound and you can route the vocals through a separate output to process further. Edge to 2100.

interface: Yammy is easier, Korg is more complete.

ins/outs: Korg wins big here. Separate outs, video outs, audio INs ... much better.

size/weight: both even here

storage: Korg wins with hard drive option

sequencers: Korg has TWO !

third party support: Yammy has an edge because there are tons of users that exchange styles and registrations on the net. The Korg population is smaller, but they DO know their stuff !

ReCap:

Both offer alot for the money, but the Yamaha is a kinder, gentler keyboard. The Korg is Steve Gadd - the Yamaha is Ringo Starr. You decide who you'd rather play with on drums. The rest of the sounds go head to head. If I didn't play so many standards, I'd still have the Korg. It's more exciting.