I prefer the forward/backwards seat and the seats themselves are more comfortable, they lean back; do that on the Overground and you hit your head on the window. Heading north you also get the 4 across a carriage seats with the luxury of a table. Plus you are more likely to get a seat on a train that has more than four carriages.
And you can often open the window. The Overground trains get horribly stuff when they are full, don't know what's with the alleged air conditioning.