Generally speaking, most people report somewhere around 30-45 minutes of trimming from the 15" trimmer and a 2.5Ah battery. It's possible to drain the battery faster if you're working it hard the whole time, think overgrown weeds along a really long fence.
As far as straight vs. hinge shaft, I have the straight shaft model and it's great, others with the hinged shaft report that it's very sturdy once bolted together. Once you have it set up I'm not sure it matters either way as the hinge is not intended to be used for storage.
If you have a need for an edger and/or a pole saw as well as a string trimmer, you may wish to consider the new multi-tool that will be launching soon.
The 15-inch trimmer (which I bought with the 2.0 Ah battery two years ago) with a 2.5 Ah battery comes in a hair under 12 pounds. With the 5.0 Ah battery it's a little over 14 pounds.
With the 2.0 Ah battery I've never felt the need for a strap. I'm an old(er) fart with about a third of an acre.
If you have other tool needs you might want to wait for the multi-tool, although it's likely to be pricier and heavier because it has a more powerful motor (2,000 watts) and solid steel drive shaft and transmission on the attachments. Available accessories include the string trimmer head, bladed edger and 10-inch pole saw, plus a 31-inch extension for the pole saw. Next year we could see a brush cutter attachment, which is coming out in other countries this year.
I have both the split shaft and the solid shaft; between the two I prefer to solid shaft as it's slightly less weight, and it's simpler design has less failure points (e.g. bolts loosening and falling out). Also, if you're a 7' tall gorilla with orangutan arms, the solid shaft would allow you to move the handle lower vs. the split which has a definitely stop point.
Between the 2.5AH and 5.0AH batteries, even if you're strong enough to hold the 5.0AH for the entire duration, it's still a matter of unnecessary fatigue. One scenario I could think of needing the 5.0 is that you wanted to trim/edge your entire perimeter, and you cannot do it on a 2.5AH without having to "return to base".
Oh, and buy the bump head version!
I’m looking at acquiring a 15” string trimmer but seek clarification on the models available regarding durability, ergonomics and gerrational improvements.
The main differences I notice between the models are standard vs. rapid reload head and straight shaft vs. split shaft.
I understand the convenience of both a rapid reload head and split shaft but are they just as durable and reliable as a standard load head and straight shaft?
Are there any other differences I should know about between the models available such as ergonomics and generational improvements?
The rapid reload head is new, but what little I've used it, it seems very solidly made. Tine will tell. I think the fact that I don't have to take it apart every time I need to reload string will make it more durable.
Apart from those two things, there really is no generational difference. The 15-inch trimmer is otherwise the same as it was when it was introduced two years ago.
But be aware, Ego is about to release a new multi-tool that includes a 15-inch trimmer attachment as an option. It's a completely new design, with a 2,000-watt motor in the handle and a solid drive shaft to the trimmer head.