This is merely related to convenience:
I have 2 E2Fs for my older DD1, and when my younger DD2 started complaining about both her Clicktight Boulevard and Chicco Nextfit, I chose to get the 3 in 1 E2F because she liked the color, I thought I could use it to start booster training the older one later before buying a dedicated booster later, and it was the babies r us trade in deal.
The best difference (in my opinion) is not even advertised, and I didn't realize it until I had both seats: the regular E2F has little fabric pockets to put the buckles into, to keep them out of the way when the child gets in. This is good in theory, but rarely works (at least for my child), so I'm constantly digging for buckles. No big deal in the grand scheme of things, and totally worth it to keep my off the chart height kids rear facing longer. The E2F 3 in 1 has plastic recesses in the seat frame that are meant for use when in booster mode. These work great to drop the entire harness into when getting the child out, and they remain in place, out of the way, when the child gets back in. They save me several minutes of aggravation every day.
The regular E2Fs are fantastic seats, but that recess is an added (albeit unintended) convenience