So I've been wondering; can Seras be a better Archer than Varric can? Seras would be able to use Rare/Epic found and Crafted Bows, and upgrading her weapons might be a little easier.
Varric can only use Bianca and upgrades for her, even though there might be crafted/Epic items with better stats and effects to them. But you can customize Bianca more than other bows, meaning their DPS might balance out.
The rest of my party currently is:
Inquisitor - DW Rogue (trying to get Assassin)
Cassandra (SnS)
Vivienne (Fire)
Of the two I prefer Sera. Early as I can in the game I go for the schematics perks, the Masterwork Ornate Bow schematic puts you way ahead of any upgrades you can find or buy for Bianca for quite a while, and by the time you get to the point where you can get arms for Bianca to put it into that damage range your pretty much ready to head to the Hidden Wastes where you can aqcuire the Masterwork spiked longbow schematic, again giving you even better damage.
As a general rule I've found upgrading BIanca to be a royal pain the keester, so much easer to just build a bow and hand it to Sera, plus there are some great longbows available through the course of the game such as the one the Duchess carries at the winter palace, etc.
So generally if I need an archer I'll stick with Sera till later in the game, Varric doesn't generally see much field time until after I go through the first couple of tombs in this hissing wastes and get the legion of the dead armor schematic. Once that happens, well then it's worth bringing Varric along because I can build him some really kick butt armor using that schematic and Silverite, which makes him incredibly survivable.