Nice comparison videos!
The 241 seems a bit stronger, but I'd still prefer the 421!
A muffler mod should bring her up to speed.
I will cast my vote for the Dolmar PS-421, but I am a little bit biased as I only ever used Dolmar IN MY LIFE.
To the OP, I have been using my Dolmar's PS-6400 & PS-7900 for everything from felling, over bucking up to limbing tiniest branches, even bush clearing - thus I understand the desire for a lighter saw even though I am built quite strong and actually own small 40cc saws. That been said, picking up a small saw wearing a "long" bar counteracts the purpose of the small saw - You will loose handling and maneuverability!
Bars can last for Years and chains cost next to nothing, thus I wouldn't set my criteria at "uses the same chain"!
As some members stated, visit as many dealers in Your vicinity and try the weights of as many "small" saws as You can.
Considering that You intend to buck with Your 661 (I might have gotten this wrong?!) and want a nimble small saw for limbing You shouldn't worry about different chain size as You'll never mount Your big saws chain onto Your small saw anyway.
Having saws with two different chain sizes will require You to carry one additional file size with You and I don't see any issue in that.
Additionally, .325" or 3/8" LowProfile/Picco chains feature smaller teeth making the limbing experience much smoother. Trust me, limbing small branches with 3/8" chisel chain can be a rough experience.
Good luck with Your choice and don't forget to post what You have chosen!
NOTE: If sharpening in the field is an issue, picking a saw that features 3/8" LowProfile/Picco would enable You to try a "PowerSharp" system. I don't have one yet and therefore don't know how it handles, but I intend to purchase a starter kit for my little 40cc saws.
NOTE2: I own two 40cc saws , and one 64cc & 79cc each. My Sachs-Dolmar 105 & 108 are identical saws with the exception that the 105 is a top handle design. The Dolmar PS-6400 & PS-7900 are also identical with the exception of cc difference.
As I mentioned before, I am well built and used to high physical load from my regular work, thus for many Years my main go to saw is my 64cc Dolmar PS-6400. Why? Because my 40cc saw just seems slow once I have to buck a tree trunk or limb strong branches, and my 79cc saw is pure overkill for most any of my needs. The only issue I have with using my PS-6400 for everything is the physical size of the saw! A big saw wearing a long bar tends to get in the way when limbing, thus a two saw plan is optimal as long as there is a logical size difference between them.
NOTE3: As pictures say a thousand words here are some of my favorite duo, 14" & 20" bars:
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Cheers,