It depends on the season and where you are buying from. Please don't turn your back on fresh berries, especially when they are in season!
Berries that you get frozen were picked at their peak of flavour and then flash frozen. (The same is true of frozen vegetables, too!) It's hard to compare with the best of the crop and handled gently for future use. In fact, I bought frozen strawberries today for a grilled sandwich of chocolate bread, mascarpone cheese and strawberry slices. Normally, I only make this dish in the summer when berries are in season, and locally grown for quick sales. But, I had a desire to give summer one last hurrah before I have to turn the heat on....so I bought frozen berries.
I'd never buy frozen berries in July or August. If you can get the berries from a good produce dealer (and not all supermarkets are good producer dealers
), then you really should go with fresh, especially if you need firm slices. Frozen looses some shape once it defrosts.
So, my point is, frozen is usually a good buy, but when something is in season, I'll always buy fresh.