2nd class is 2+2 seating. 1st class is 2+1, with proportionally more legroom. Both are equally comfortable.
1st class usually cost about 50% more than 2nd class, but sometimes (weekends etc.) you can get cheap upgrades to 1st class. Personally, I would only go 1st class if I got a good price.
It doesn't really matter where you sit in the coach, or which coach you choose. Some people think it is an issue to be travelling backwards. If you reserve a seat in advance it is not always possible to tell if that seat will be forwards or backwards facing, you have a 50% chance.