Leg-room may differ by only an inch or two, so if it is for extra leg-room, probably not worth it.
Note that WE offers same seats in Plus and Economy, but Plus has the middle seat blocked in the same fashion as European business class tends to be. You could argue that extra leg-room can be had be stretching into the empty middle space though.
I am not aware of WE offering paid upgrades at the airport though - have always only booked Plus whenever I have had to take WE.