I've been in the same position about 4 months ago. It's fall here and the temps are dropping now but still day times are in the 20C's and still sweaty if you hammer hard.
Well when the summer started in Canada (I'm in Toronto and yes we can hit high temps dispite our stereotypical views on our winters
) we started hitting 30-45C temps here and that's with the humidex. You walk out and bam it's in your face. Well I did not have panniers as I like to move with my gear and panniers I've found you can't really lock them up making them more thieveable (sp?) if you're doing stop and go shopping or a movie and such later or locking it outside. I use a Deuter AC28
for my general purpose bag which hold 28L internal and can hold two water bottles outside, concealed rain cover, the side compression straps can also air in keeping the rifle barrel standing if you carry a carbine/short rifle, internal 60/40 split divider which you can fully unzip for 100% use of the bag, my store carried two tone blue or two tone grey colors to pick from, suspended mesh back, hydration compatible and can take a 3L bladder (or put your Ulocks in there for center balance of the bag), and it has side loops for poles and such as it was designed for a hiker but because of the suspended mesh back any cyclist will greatly appreciate it as it's almost like nothing is on your back covering the wind from cooling it down. I have loaded this bag out to ~35-40lbs and did not see any signs of fatigue on the straps or seams after a week or two of useage which tells me the bag should be able to hump 40lbs no problem tho you may want to take 5lbs out if you want to take some stress off. The lower 40% of the bag (which if you look outside the bag that would be the lower zipper) has two zippers that you can position in the middle making putting a blinkie there easy without falling out unlike one way zippers that don't give you any room to put a blinkie unless you mod the bag.
On the front of the bag you have a velcro'ed loop for hold the hydration tube but for those that like thier gadgets or pouches you can use that loop (horizontal) to clip a mp3 player/radio w speaker/FRS-GMRS radio comms/cellphone/PDA/camera there for quick access. The rest of the straps for attaching stuff are all vertical so you'll have to get a vertical pouch or make something vertical. The bag has chest and waist cinch straps to keep you locked in.
I also have a Deuter Race Exp.
for light trips where I'd want to have some bag to carry stuff. It has an expandable 3L zipper should you need to carry more stuff but zips up to give a slim small profile. This bag features the unique suspended open mesh back of the AC series, chest/waist straps, blinkie loop, concealed rain cover (if memory is correct reflective) , concealed helmet cover (non reflective), reflective rear and front strap logos for low light/night visiblity, hydration compatible, hydration hook on strap (no hortizontal velcro tho
), and side mesh pockets for quick access items. Most I think I've loaded this bag was ~20lbs but I'll be conservative and say ~13-15lbs and did not notice any stress on the straps.
Both bags are excellent bags, holds up well to city/trail/offroad/curb-pothole jumping (realworld tested by me), for the style people the bags are stylish in looks with all the functions concealed outside to the normal person making it look slick when you need to use them, for the function people both bags (more the AC28) are fully functionable with lots of room and outside space to carry extra unexpected gear, for those like me that sweat rivers in the heat the open mesh bag really dumps heat. I've rode with both bags and without the bags in the 40-45C temps and I'll probably say with the bag you get 75-80% air venting where as without the bag you get 100% but using the old bags without this mesh suspended back you end up with probably ~20% or less air venting in the back.
If you use a cycling jersey or any jersey shirt you'll probably get another 10% venting on all the numbers. I used to use a microfiber shirt with wicking capabilities thus why I gave those numbers above in my experience but once I got into cycling jerseys with something like 100 perforations per square inch I noticed a difference in temp cooling on the body as more air passes the shirtand all the tiny holes pulls the moisture out.
The Deuter bags have been recommended many times before on the forum and that is why I got them after reading up on it, asking about the features and checking out my local shop for the bags I fell in love with the functions and also slickness of the Race Exp. bag with the helmet cover and rain cover all concealed which no one knows it's there but I have used them both before and people seemed amazed how slick and quick I was able to take them out and cover the bag while everyone else was running for cover. The helmet cover is not some one piece sewn in item it has two hooks which you hook tot he side loops to secure the helmet. This is good because it allows you to position the helmet the way you want then have the helmet cover stretch to conform over the helmet to hold it in. You basically hook one side in, put the helmet in, then hook the other side on the bag. Removing is quick.
Both the bags rain covers are removeable should you want to hang dry them else where. I forgot to mention the AC28 rain cover and bag does not have any reflecto material and that you will have to add yourself. For those that like concealment/camo because the rain covers come off you can take that to a sew/fabric shop and get some camo material and have the shop use the original cover as a pattern to make a new one.
Oh yes the website for Deuter is here
Edit: I live in a semi-hilly area. I am always forced up a hill to get back home (I live on a hill) and thus some what surrounded by minor and large hills (~35-45degree are my two local steep ones blocks away) so take take that into consideration with my input on the bags and venting. On the flats it would be cooler for sure.
If you get both bags get the grey two tone color. It compliments both bags and the big/small bag combo gives you the best flex combo but if you only had to make one purchase I would go withthe AC28 for it's general purpose use/function/space. You will soon love the bag and want another one one you try it and know why.