Here is my take. Strung them both in identical racquets, and was able to switch back and forth during play.
What I am describing mostly pertains to the time when the strings have settled ~ 3 days after stringing.
[ When I hit with Firewire the first day, just a few hours after stringing, there was nothing special. Felt like an ordinary poly. The character developed later. ]
Strings were not used "pure": Firewire (51 lbs) was paired with Red Devil (-4 lbs). Big Spin (54 lbs) was paired with Head Sonic Pro ( -4 lbs ).
These strings do not come through TW, so no data exists on their stiffness, or friction coefficients. It is hard to figure out what should be paired with what.
Both strings are triangular. That's about all they have in common. In all other aspects they are very different.
1. Feel
Firewire is a clear winner in feel. It has a rubbery / stretchy feel, which is a white unicorn among polys -- extremely difficult to come by.
Extremely satisfying. Very long dwell time and pocketing.
Big Spin had a comfy feel (by polys standards), but no noteworthy dwell time or pocketing.
2. Power
Firewire was a clear winner on power. The difference between it and the Big Spin was about ~5 feet in baseline terms.
This difference made itself very pronounced on serve. Firewire produced a much juicier, heavier serve and ball action.
It was easier to "grab" the ball for the kick-serves, and the way it curved was kind of unique.
3. Stability
The Big Spin is more stable string. It played the same on day 1, and on day 3. It did not exhibit much change at all.
Firewire started out "stiffish", then softened up considerably.
4. Spin
Both strings are very strong here. We are talking RPM-Blast or Black-Widow levels of spin. Both are extremely grabby.
Firewire took the upper edge. Maybe it was the dwell time (and not the edges) which helped, but this string can produce spin wonders on the court.
The Big Spin was strong, (stronger than regular polys), but a level below the Firewire.
5. Overall
Both are top-notch strings, rich in character and feel. I would classify Firewire as a powerful, soft and lively poly with tremendous spin.
It has its drawbacks: it changes a lot during its life. It has a tendency to become "wild". I noted myself that I need either higher tension for the next time, or I need to pair it with a stiffer cross. While it was interesting and entertaining, esp. to the audience, I actually preferred the Big Spin ( and had to finish the match with it ).
While less spectacular than Firewire, not as cushy a feel, not as crazy a spin, etc. it was much more consistent. The response was more linear and predictable. I felt more control. It aligned more with my intents and expectations. I was able to play not a spectacular, but well-scripted game with it. With Firewire, I had a few "wonder shots", but also a fair amount of "wild" shots and other "unexpetancies". I love the feel of Firewire. My search will now shift towards finding a suitable cross. If anybody has suggestions, I am all ears.