I have no definitive preference; and, I am not sure if I ever will have one. Were there to be one-single feature as a dealbreaker... or, where I to be able to compute weights of each issue and a rating, to have a sum-product rating (for those math folks), then, perhaps I could arrive at a numerical score...
Here is how it all "shakes out" for me, all subjective opinions;
E-P1 advantages
1. styling, I like the retro look. it was my first m4/3 and I will never forget that first moment holding it 17-June outside B&H.
http://forums.dpreview.com/...forums/read.asp?forum=1041&message=32167320
2. JPEG colors
3. AF with Oly lenses
4. in-body image stabilisation; I have a bunch of MF lenses.
5. rear grip, entire side edge is curved
6. front grip's comfort
GF1 advantages
1. faster, and better autofocus
2. on-body flash; excellent for fill flash
3. click-dial over the E-P1's dial and wheel. though I really like that vertical dial.
4. one-button instant video
5. front grip's torque.
6. aperture ring works with my PL 25/1.4
7. it takes a EVF, though I don't have it, as I hear it is not so terrific.
8. menu system is easier for me.
9. auto-focus assist beam
I have not yet tested each LCD screen in direct sunlight as a comparison. I am not bothered by the resolution of the E-P1 digital PEN's LCD; I believe Oly when Mr Watanabe says they were concerned about refresh rate and direct light visibility.
I "view" cameras as unique among all other consumer electronic instruments. The reason, for me, is that the tactile relationship tm one's hand, and the emotion of the activity, is a completely unique experience. Then again, I "fondled" the E-1 for three-months at home in the evening, while watching TV, just to get to know it. The depth of that grip and the scalloped thumb grip on the back.... no one has come close to repeating that. I can hold the E-1 with my finger tips down by my hip gunslinger style. I love the weight and feeling of my Contax TVS III titanium film camera. I just enjoy the feeling of Nikkor's smooth manual focus lenses.
Before anyone just counts the list, they are not all equally weighted, for me.
Were I to not have an investment in Oly 4/3rd lenses, then, I would likely would have gone for only the GF1, for the faster autofocus, and built-in flash... at the "cost" of shooting RAW and post-processing, relative to using the E-P1's OOC JPEGs.
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'Photos are what remain when the memories are forgotten' - Angular Mo.