I believe everyone is suggesting the extract, causing using fresh berries, especially raspberries, can be very tart and take awhile to mellow out. Extract can just be added and ready to go in a couple weeks.
I have done both and the extract gives a artificial flavor. I prefer fresh Raspberries or Oregon Fruit Puree. IF you go fresh I recommend #1-#2 per gal.
There are lots of other raspberry wheat recipes in the hbt recipe database, its a great resource - I use it all the time, even if its just for inspiration.
With fresh, i would suggest using no more than 1lb/gal, since they're a "strong" fruit. My the ones i have made at 1lb/gal or greater are too heavy on the raspberry flavor.