Well - lots of mixed advice. Here's mine.
I don't bike camp but I guided in the mountains for years (Seattle Mountaineers) and after several seasons worked out my favorite sleeping deal: a down, mummy-style, sleeping bag plus a bivi sack - that did well in over 20 years of guiding.
1) down is the warmest and most efficient. My bag goes in a compression sack that packs to just 8x6 inches so it takes little room in my ruck. Avoid at all costs synthetic fill of any kind mostly because they are huge, take up half your ruck, and are heavy. The disadvantage of down is that it loses its warmth if it gets wet whereas synthetic dosen't.
2) I use a bivy bag because it is much warmer than a tent, lighter than a tent, and also takes up little room in your ruck - less than my sleeping bag. The bivy adds to the overall warmth much more than a tent because of the smaller space. And it keeps the down bag dry. Lastly, if it is a warm night, forget the sleeping bag and just slide into the bivy. The only advantage of a tent is if you are using it for more than one person - and you get the other guy to carry it.
3) Ratings: I pay attention to the ratings that the manufacturer puts on the bag. Unlike the other posters I have never done field trials of bags and have only purchased two. The first one was rated 0F. It did well - too well (hot) so I sold it. My second bag was rated 30F and it has keep me toasty to below zero in the mountains - inside my bivy bag.
I should also also say that I always carried a blue foam mat. Any mat is bulky but I creased-cut mine and used a heat *** so the thing could be folded mostly flat instead of rolled. I then carried it inside my ruck next to my back. They only lasted about two seasons at most but they are cheap.