You can't go wrong with the 5105. I've had mine for about two years now and I like it a lot. I heat our house with wood and we burn about 8-10 cords a year. I've cut everything from 48" Red Oaks to 20" Shagbark Hickory to 40" White Elm to 10" Red Elm.
I used to cut firewood with two really sweet saws: a Stihl MS460 for bucking and a Stihl MS361 for limbing. The older I got the less I liked that setup. Too much equipment to drag to the woods and too much weight to slug around with through the timber. Although my arms were a lot bigger when I was doing that.
I am now on the one saw plan and it is serving my needs quite well. I've got 16" and 20" bars in .325 pitch, .050 gauge. Run Oregon 20LPX on those. And I've got 18" and 24" bars in 3/8 pitch, .050 gauge. Run Oregon 72LGX on the 18", and 72JGX (full skip) on the 24". The 24" is a lot of bar, but the full skip makes it doable and actually fast enough for my needs and the limited amount of use I give it. It takes an extra minute or so to swap out rims when I change from one pitch to another, but no big deal.
Lately, I've been running the 16" and look for trees to cut that don't need more bar than that. If I'm feeling pretty run down, I'll throw on the 20" so I get some extra reach without having to bend over so much when limbing out a tree on the ground. 18" 3/8 is a really nice saw/bar combo for the 5105 and I use it quite a bit to quarter the huge rounds before dragging them to the splitter.
I'm not a saw collector; I have a saw for three reasons: My family and I like wood heat, I really like cutting firewood, and I enjoy teaching my sons the value of hard work. For me the one saw plan with multiple bars works well. It simplifies my already complicated life. I've got less maintenance, fewer spare parts to have on hand, and less stuff to take to the timber. I don't care about a few milliseconds saved on cutting a 3" limb. I'm happy with what I have.
Just my two cents -- I realize that not everybody has the same priorities.