Most bags rated at around freezing are good. I have heard several times that the sumerlite was a bit slim. It doesn't adjust as well with interior layering as something a little fuller cut. IE with thicker layers inside, it can cause loss of lofting and insulating value. Soo, at a guess, there are faster diminishing returns with a Summerlite than with a larger girthed bag. Depending on your body size, this could cause you to choose differently.
But, I think I would choose the lightest GOOD bag rated at or about 32F. I am often out in fall, too, and I use a 40F rated bag and been down to 25F with layering (down jacket, smartwool mids, wool socks) and a Nightlite pad. Colder I wouldn't want to go.
The center zippers can be a problem. Especially at colder temps(<20F.) Because the draught tube does not tend to hang over the zipper, as with a side zip, I believe it will be easily displaced. A fromt zip would need some kind of velcro to keep it in place and working. The extra seaming with the zipper/draught tube could also lead to some cold spots right over my core. Probably fine to around freezing, though.
Quilts start loosing their appeal at around 32F. Though there are some that swear by them at colder temps. I find them to be quite draughty with that blast of cold air as I roll waking me in chilly temps… Nice at warmer temps, though. Often the wife and I will zip our bags together on top and use them as a quilt. Together, we have been down to ~20F with the 40F rated bags, comfortably. At 32F just a good pad and hat is all that is required to use a quilt. I often bring a knitted cap I can pull down over my eyes and ears, otherwise they will come off. Being somewhat thin on top, I resorted to this over the past few years. With a quilt like setup, I often use a neoair AND Nightlite pad.
"Using" vs "Planning to use" clothing to stretch a bags rating is iffy. If you are planning on below rating temps, this is likely a mistake if you have not purchased a bag, yet. Generally, weight vs thermal efficiency are lighter for the warmth with a bag rated at the planned on temp. Dan Durston (sp) wrote a good post on this a while back. (hmmmm, I don't find a link offhand…) A 7oz jacket will only supply about 2oz of down. Overstuffing 2oz will cost less, work as well for sleeping, and be lighter. You can extend this to general application to include a better bag to start with and eliminate the jacket. If you need the jacket, anyway, then include this in your girth measurement, add 5-10F to the bag's rating and bring both a bag and jacket. Not wearing your cloths in a bag at night will remove any insulating value. BUT, if they are damp from sweat or water, you should not wear them inside the bag. I NEVER plan on hiking cloths as sleeping cloths, it is too "iffy" to be reliable. Dry socks and dry long johns (an EN rating test model) goes in the pack with the bag. Sh!t happens, though. I have had to wear them once in a while for hiking.
Doing this allows me to save money on the number of bags I need. I have two bags: a 40F and a 0F. I don't need a 20F bag. I don't need a 30F bag. I simply use the 40F bag, and layer as necessary, though I pay about 8oz-16oz in additional weight at those temps. At <20F, I use a 0F bag, too warm at 10F, but with a jacket and layering good to about -10F. So, I skip a summer bag (>40F), a cool temp bag (between 20F and 30F), and a very cold bag (0F to -25F, I use both bags together.) Again, YMMV. But, I pay in extra weight at those temps…as much as a pound or two greater than what is necessary.
Note: I open my 40F bag into a guilt for >50F.
Not to start some arguement, but quilts often require a bit more ground warmth because they don't "seal" like a bag does. So, a heavier pad is needed for me. YMMV. The trade off in extra weight between a bit more pad and a heavier hat, and, a full bag?? That IS the big question…Your answer will be based on your own conclusions, often testing is the only way to determine this.
