What an interesting pistol build, taking Fin->WK after the primaries. I can understand wanting to super-mod your pistols, but the chance of getting the kinds of mods you want are often pretty low. In most cases, A/B aren't enough of a boost to warrant so many invested levels, and T's speed increase is kinda low by the time you have SoGx3 and Finx2 (which is already 0.28/shot): this leaves P as the mod of choice. The odds of getting P3 for BOTH pistols by Cyberdemon is already low, not to mention getting it at a high enough floor to really come in handy. This is the reasoning I typically use to choose SoB over Fin->WK when using a high-offense pistol build, although both are achievable in the same game on HMP+ difficulties (just barely on HMP though).
To compare it in simpler terms, note that each P gives the pistol another d2 die, which (on average) increases your DPS by 1.5. SoB adds a +1 to damage on any ranged weapon, which also increases your pistol DPS by one. By playing Fin->WK, you hope for enough power mods to make the investment worthwhile. I could write down the exact gains in sheer DPS (damage as well as firing speed), but it's not really important unless you want to optimize your build.
The biggest reason to want damage on pistols is that they have the capability to cause knockback quite reliably. Using the extreme example, SoBx3, SoGx3 and a P3 pistol gives 2d7+6, averaging to 16.5 (range is 13-20) and giving you 100% knockback with that gun. Now, SoGx3 and P3 only is 2d7+3 and changes to 13.5 (range is 10-17), while SoGx3 and SoBx3 only is 2d4+6 and changes to 12 (range is 10-14). Both of these have a 50% knockback and, when applied to both weapons, basically ensure that both pistols will get at least one tile of knockback (the ideal amount). The clincher is that your 12 damage is guaranteed on the SoB track and not on the Fin->WK track, and knockback is extremely important later on for survival (since you're sacrificing DPS for defense).
Alternatively, MGK blocks SoB, forcing you to pick the Fin->WK route. It's not a bad choice, and if you happen to find any early unique pistols I would take it...the damage output of the uniques is vastly superior to any SoB levels, particularly the Trigun.
I take it B2T2P1 pistols were good enough to sweep Phobos Hell HNTR, though. Just keep in mind that there are some certain situations (Arachnotron caves, for instance) where knockback will save your ass hardcore.
EDIT: Sorry, messed up a bit of calculating up there. DPS gain for a d2 die is 1.5, not 1.