mobiledynamics...
I think the important thing is to go in the store and try each of them out. Pull em down off the rack, or get an HD employee to unlock them so you can hold them and swing them about personally.
BYOB (bring your own battery)... not for running the machine, but for feeling the weight. You mentioned not minding going with the lighter M18 system over the DeWalt 40v... but the weight and size of the battery alone is not enough to judge when dealing with a 6 foot long stick that has a spool of string and maybe even a motor on one end.
For those models that do have a motor on the cutting end of the stick, a heavier battery on the top side of the stick may help counteract the fatigue that can come from having to keep the direct drive motor AND the spool of spinning string elevated above the ground.
Already, the weight is biased downward due to the unequal distribution of the 6' long stick at the holding point, which is generally at 60/40 or 66/33 ratio above and below the grip point. Many of the cordless string trimmers, and certainly ALL of the homeowner / consumer grade string trimmers, have a direct drive motor at the string head, which adds more weight on the 66% end. Combine that with the absence of a big motor and tank of gas at the top end of the stick, and you might wish you had a heavier battery at the top end to counter ballast the bottom end.
Items held out of balance from their center of gravity can seem heavier than items that actually ARE heavier, but held in balance. The difference is like holding a five pound weight by the edge with hands outstretched, vs holding a 50 lb weight by the center with arms close and bent. So weight alone isn't the deciding factor... the key is in the distribution of the weight, and with long items, the center of gravity in the operation position.
With the above being said, I am not advocating yellow over red or red over yellow. I am simply urging you to try them out in the store first, and if your kids or wife or grandma are the people you expect to actually use the thing, have them come with you for the audition. You are fortunate that yellow and red are both carried in stock by Home Depot, so you can easily hold auditions.
My brand of choice isn't carried by Home Depot, nor Lowes, nor any local tool, lawn, garden, landscape, lumber yard, or home improvement store anywhere within one of the largest metropolitan areas in the USA, which makes it impossible to try them ahead of time to precheck ergonomics... unless one happens to catch a periodic road show that might roll through a neighboring city every 6 months or so, provided one is willing to drive an hour and a half to get there. But the quality of the product is worth putting up with the lack of immediate availability.
I considered the general principles of string trimming... by observing what the professional mow blow go guys use daily. Straight shafts. Loop handle. Small guard (for rotating the string orbit vertically to edge or dig those little weeds popping out of sidewalk cracks). I observed and recalled how having a light head made it easier to float the working end of the tool back and forth in swath cuts through a field of weeds.
I also considered the string changing process, comparing, through my own life experience, how much easier it was to flip a straight shaft string trimmer upside down, where the engine resting on the ground stabilized the entire shaft now easily presented to me at eye level height for string feeding.... as opposed to the curve shaft corded string trimmer, with motor on the cutting end, where I had to sit down somewhere to deal with changing string, as the top end was only a handle.
Another consideration is ease of cleaning the tool after the work is done. I don't know about you, but by the time I get around to weeding, the weeds have gotten so tall I might be better served with a backhoe, only a tractor can't fit through the gate. So needless to say, the working end of the string trimmer is covered in green muck. With no electrical motor directly above the string spool, I could freely rinse the working end without fear of water infiltrating the motor. On the model I had with the electrical motor directly attached, it took a lot longer to clean that tool, so as to not cause problems with the motor.
On the other hand, a directly driven spool of string is more mechanically efficient than a shaft driven spool of string. Mechanical efficiency can correlate to electrical efficiency, which can correlate to longer battery life. Or, just change batteries. Endless power. The shaft offers a mechanical "delay" fuse between the motor and sudden resistance in the cutting head. Split shafts offer a variety of other implements to make full use of the power head, like articulating hedge trimmers, extended shaft pole saws, rotary brooms, rotating cultivators, etc. Lots of things to consider.
