Depends which airport in Canada. Last time I changed in Toronto (Atlanta to Tel Aviv) I had to go through passport control to enter Canada, and then go back through security, into departures. I'm not sure if I had an ETA though, I forget.
I went one December and it snowed ....was coat and gloves weather walking down the Strip in the evening but nice in the daytime
Everyone is different I guess and if I was to go back it would be Dec or Jan during CES show. I went over Xmas and it was warm enough for a t-shirt during the day but you needed a coat at night and I had a great time. I guess if you like to party in a pool during the day then you don't mind the heat.
I've been there in mid December and it was bright and sunny during the day but air temperature was low. Once the sun set it was very cold just like January in the UK. Still pretty busy though. I also visited San Jose, Palm Springs, San Diego and Los Angeles at that time and they too weren't warm.
It’s all about cocktails by the pool...