Both are highly rated. I used the 2012 Murmur intensivly last year logging about 4 week long trips with it and a long weekend...about 47 nights all told...
1) VERY comfortable.
2) VERY efficient...Pad pockets kept my sleeping pad, saving internal volume. Rear Pouch was plenty large for any expansion (this was also a problem.)
3) Can fit a bear-ball.
4) "Just" fits two weeks of supplies, though it is overloaded.
5) Rain flap worked well.
Dislikes (note that I have communicated this to Grant, newer ones are corrected.)
1) Strap Slippage on the first ones, slick, nylon straps tended to move slightly within the buckles, over time, loostening them when walking. Corrected.
2) Front Pouch was too big. If you put a log book,fuel bottle, extra food in there it sagged below the bottom of the pack causing a LOT of abrasion when setting the pack down. Corrected.
3) 2nd run had some mis-placed shoulder straps. Also, the seams were not folded/rolled but simply stiched, these pulled through the fabric leaving a frayed edge. Corrected. I would also state that they had some trouble with the person(s) doing the sewing for them (shoulder harness and seams) and no long works for them.
The Murmur easily handled 23 pounds at the beginning of two week trips. It was not uncomfortable at all. 'Corse, it is hard to find anything uncomfortable about 20-25 pounds on your back. It seems small but it also includes pouches in the size figures.
The MLD Burn is a much larger pack, the main body will hold 2200ci. Really they are not comparable. After adding in the side pouches/front pouch it is a quarter bigger. After adding options, like a lid/pouch, it is more than that. It is also more durable being nearly full DyneemaX. It is also heavier and not as efficient, lacking an external pad keeper/pad frame. If you like pouches, this is the way to go. Top lid, waist pouches, stow pouches, and hydration sleeve are addons! A complete pack will be about $285 and weigh about 17oz and can handle about 3000ci. But, because of the internal pad keeping (yes, they sell the internal Klymit frame, too,) some of this volume is lost, taken up by the pad/frame.
The MLD Burn is twice the pack, but is also about twice the price. It is less efficient than the Murmur but has better overall durability, but this is a judgement call. For day trips, quick overnight trips, I would highly tend to the Murmur (knowing any failure would not put me at risk) so the lighter weight (~9oz) would be my choice. A striped Burn comes close, but the lower carying efficiency is the clincher. As you said, "The Murmur appears lighter, while the burn sturdier." But, this does not consider load carrying efficiency (external pad keeper.)