I'm generally happy with doing my own research and booking online. I prefer through the airline rather than some amalgamator, though. My wife and I are flying to Birmingham in a few days. Ostensibly w/ Delta, but it's actually KLM with a code share. Seat selection was not available online through the Delta website, but I simply called customer service and got seats. I looked at the seat map online while the clerk offered options. Worked well.
Minding the T&Cs is indeed important. I get two free 70-pound bags per person on international Delta flights. My wife and I will be taking a train from Birmingham to Edinburgh and flying Lufthansa from there to Frankfurt. The ticket class I picked lets us each take one 50-pound bag. Free bags on DL could become very expensive ones on LH! Winter in the UK and on the continent requires more layers of bulkier clothing than summer travel (Plus, no glider flying while in the UK) so careful planning will be required. I expect we'll have three checked bags between us, so we'll only have to pay for one on LH as long as I keep the others below 50. It's good to know these things in advance.
We'll have plenty of surplus capacity for Christmas Market detritus on the way home. (Train from Frankfurt to Stuttgart and flight to Atlanta from there.) Totally off topic, but flying westbound from Europe to the US totally bores me to tears. This will be my 56th trip across the Atlantic.