Well, I’m not trying to sell you on synthetic. I like both, for different kinds of trips. Here are the points to consider:
Tents really don’t provide significantly more shelter from most precipitation events and outisde moisture than a tarp/bivy combo (tho tents do have other benefits), so that’s seldom the deciding factor. Tents can be less breathable, but if yours has plenty of ventilation (mesh), and you don’t seal up the tent when you don’t need to, that shouldn’t be a problem either.
If you really are talking about trips as long as a week, then the deciding factor is mostly the overnight conditions and temperatures (regardless of the temp range of the bag alone). 1. If its mostly above freezing, and/or moderate humidity, than down is a great choice. Of the brands you mentioned, I’ve seen many excellent down bags by Marmot. 2. If temps are ~consistently~ below freezing, and esp. if the air is damp, then synthetic would be a safer bet, at least until you gain more experience. TNF and Marmot both make good synth bags. Again though, the shorter the trip, the less danger of a down bag going flat due to accumulated condensation.
Key questions:
1. Where will you be backpacking most of the time?
2. Are you thinking 20F range for extra security on trips that will mostly not be that cold, or are you using clothing to go colder than 20F?
Using a lighter bag (rated to average temps) + clothing (to extend into coldest temps) is almost always preferable–not just to save weight, but for comfort. Getting totally undressed on a chilly evening before hitting the hay, just so you don’t swelter inside a 20F bag on a 35F night, is neither efficient nor comfortable. Reversing that scenario on an even chillier morning is much less appealing still.
If you’re going the ‘added security’ route of a heavier bag, then I could run circles talking about how a down bag that’s warmer than it needs to be still isn’t so warm in the worst case scenario (if it gets soaked), and a bulky synthetic bag that’s warmer than it needs to be ends up being even bulkier!
But, if I had to choose between those two options in an overly-warm bag, I’d rather enjoy the lightness and compressibility of down on a daily basis, since ‘worst case scenario’ is more often the spin of some marketing exec than it is a real-world situation.
Sorry if this seems overcomplicated, but talking in theorieticals usually is. First things first. Tell us about yourself: where you’ll ~mostly~ be backpacking, what the temps usually are that time of year, and what your clothing setup is like. Simpler advice will follow. Cheers Cody! (Hope you’re not fed up yet!)