I cut orange and avocado orchards, initially as a firewood business, now just for ducks. Some of the bigger trees will get up to the size you are talking, especially the avocados. Long ago, I realized that there was a need for two saws for comfy and efficient cutting...so I got a Stihl 025 and an MS290. For most of the work, the 025 is what I use...yes, not 'pro' quality, but that saw has NEVER given me one problem! But for about 20% of the work, it's a little light, so the 290 comes into play.
Could I make do with one? Maybe...but if I did, I wouldn't be using the best saw for the job, cuz the 025 is much handier and lighter than the 290, and the 290...now a 390 with a recently installed big bore engine...is so much more powerful and has a longer bar. So with one saw, I'd be compromising, I figure, about half the time.
Something to think about if you're gonna be doing a big job, and continue on cutting firewood. For me, the 'farm and ranch' range has worked well. I use Stihl cuz I drink the Kool Aid...and believe the Kool Aid, although I'm certain that Husqvarna is probably it's equal...plus there's a local dealer in my town. That last is the more important of the two.