Sera require more micromanagement to be at her peak. However, she does a fine job of chugging flasks and shooting arrows by herself. The DPS of a tempest is very impressive, and also very amusing to watch. Sera will go berserk when she uses the lightning flask. The dps is strong during those bursts, but rather lacklusting otherwise. She is also easier to gear up. You can craft her bows and upgrade them, or find things as drops. As Varric you are limited to, the otherwise awesome, Bianca. You can get bows with interesting fade-touched effects for Sera, while Varric can't (I believe.. I havn't seen if any of Biancas upgrade can gain these effects).
Varric requires less micromanagement to be optimal, as his dps is not maximized during set intervals (I.E. Sera during flasks). The passive of Artificer is also rather amazing for the group, hosting a damage increase and crit chance increase. His dps is sustained, and very solid, due to the passive that lower his CDs by 0.5s for every crit done by his allies(and himself). If you get him some good crit chance and the 10 stamina per crit in the poison tree, he will chain long shot and explosive shot to no end.
Which one to choose? Depends on your team. If you build a lot of crit chance, Varric goes ahead. If you don't bother with making your whole team a crit-machine, Sera gets better. It also depends on what you want: If you spend long time in fight, Varric is better. If you finish them in seconds, Sera does the best.
Overall I feel it's a marginal difference. Varric for team buffs, chest hair and Bianca. Sera for crazy-chick, super amusing-speedy-attacks-during-flask and higher burst.