firstly just to point out that there are quite a few other options on the market (enlightened equipment, nunatak, jrb ...to name a few) although no doubt the companies you mention make fine gear.
In continuation to what was said here (bags vs quilts and how to sleep with a quilt etc) A few notes:
1) temp - these are all male temp ratings. Female ratings are WAY WAY lower (take a look at a comp like Marmot that has EN rating and women specific and you will find out that there is about 10F diff) So if you REALLY intend to sleep @ low 20s you need to really be aware of this
2) take a look at Mammutsleepwell to learn a bit more about EN temp ratings. Although they have their drawbacks it really the only system that can be easily used when comparing diff bags from diff users.
3)westernmoutaineering and featheredfriends are the golden standard in US bags and are known for being "conservative" - which just means they are true to their rating. From what i have garnered here most people think Katabatic and Enlightened are also rated well (see below how to compare).
4)zpacks and nunatak are NOT well rated - just looking at the down weight content will reveal this...most people buy them with overfill....and even Joe (and Tom) in private communications with me have admitted as much. To make it blunt - a healthy "normal" man can very well sleep NAKED in a WM 30F bag @ the rated temp while he will prob need some insulation (eg - socks, base, vest whatnot) to do the same in a zpacks)
This is not to say these are not fine bags (I personally have 2 Nunatak bags) - just need to be aware and fill accordingly
5)without getting everyone wound up again (emotions run high when down is discussed) it is now common wisdom to think that down weight...or rather effective down weight per surface area (since temp loss is proportional to area exposed to cold air) is the dominant factor determining bag warmth - so when comparing I would NOT really heed the mfg temp rating (unless its EN) but look at surface area (hood yes/no, girth and various places etc) and down quality and weight. If you do this trick you will quickly discern the conservative ones (WM, FF, Kata, Enlight) the normal ones (MB, MARMOT, TNF) and the "spartan' ones (zpacks, nunatak)
6)Secondary factors are loft (which basically is just the baffle height at the end of the day unless under filled) and design - eg cinch cords, draft control, efficiency of space inside bag and footbox etc
7)when you think about layering inside the bag - make sure you understand the relative effectiveness of your clothes. Take a look at Richard Nisley's "paradigm" post where you can see the relative CLO/TOG/R values of different arch-typical clothing systems. Unless your down jacket is super puffy it prob isnt much warmer than a 300 wt fleece. and thus will help...but not to the extent of giving you an extra 30F (think about it - you are looking at 40F MENS bags.....some of which are optimistically rated even for men...then you want to take them down to 20F....so in a WOMAN's terms thats like taking a 50F bag down to 10F almost!!!!!!
Take note of the down weight of your jacket...and assume say 50% of it is under you and you will soon realize that it probably wont cut it.
8)If you plan to use the quilt in moderate climes (say above freezing 99%) I would go with a TRUE MENS 20F quilt and assume your clothes will get you to 23~25F at a push
On the other hand if you plan to regularly go down to 20F I would go for a heavier quilt or as a compromise get a really warm jacket. Another thing to think of ($$ aside for a moment) I try to think of layering as using the clothes I already realistically take with me on that specific hike. so in hikes where night temps are 20~30F I would not normally take a full blown down parka...cause day temps are above freezing....so my light weight (eg MB EXLIGHT, PATA micropuff etc) puffy is all I will have. Taking much heavier clothes as a proxy to a warmer bag is less weight efficient. that is not to say of course its not a valid option if you are optimising $$ and cant afford more than one bag