I have had both an E- P1 and a G1, which I updated to a G3 and an E-P3, so I can compare first hand and here is
what I have to say : the E-P1 is the only camera you should avoid for the following reasons :
1) they don't have the plug allowing you to mount an EVF;
2) their display screen is of mediocre quality; you won't be able to frame your pictures in good outdoors lights.
3) the first kit lens was of very mediocre quality;
4) there was a problem with the IBIS when you shot near handhold able speeds, around 1/60 and 1/100s;
5) the AF is slower than on later models and than on the Panasonic. It is OK to shoot landscapes and still scenes, but
not to shoot at playing children;
I don't use that body anymore, I didn't even give it to someone else (that would be a way to disgust them from
photography). Nowadays I'm just using the battery as a spare for the E-P3 which is a much better camera. If you want
the Olympus interface you should consider an E-Pl1,
The G1 inspire of being Panasonic's first model is still a very good camera.
1) It has a lovely fully articulated LCD, which allows you to shoot from unusual angles of view;
2) it has a great viewfinder, allowing you to shoot in bright light much more easily;
3) it comes with a much more reactive AF;
4) it has a very highly regarded 14-45mm kit lens, very sharp and with great local contrast;
5) on the down side, it has the same sensor as the E-P1, but the out of camera jpegs aren't as good as those of the
E-P1. That IMO is not a reason to get the E-P1 over the G1. If you want Olympus nice Jpegs colors, get the E-Pl1/2
which have the EVF plug : you will need that EVF ! Go for the E-Pl2 rather than the E-Pl1 if you can : it has a better
kit lens.
6) other downside, you won't have IBIS, but OIS on selected lenses, included the kit lens.
Good luck with your purchase ! M4/3 are great cameras and except for the E-P1, you can't go wrong with any of
them.