confused about code shares
 

Hello!

I'm hoping someone with a whole lot more experience than me can provide me with some guidance!

I am booked on Lufthansa (Barcelona to Washington DC). It is a codeshare and the real flight is on United. I wanted to reserve seating so I went up to United and selected my seats; about a week later I was up on the Lufthansa site and saw a different plane configuration and my seat assignment was different.

Seeing two different plane configurations sent me up to seatguru where I plugged in the flight information and got a third configuration!

All this info for two simple questions!

1. When I do my on-line check-in, should I just check-in with United?
2. Should I be concerned about the difference in seating assignments between the airlines? Will I get the United seating assignment?