as much as I love the Nebula, I must say that the weaver has supplanted it as the defacto sci/cruiser hybrid.
They're pretty close.
The Orb Weaver (with Universal Tac LtCom) is a beast. Compared to a EPTW/EPTS Nebula, a Weaver has an extra Tac Console slot and an extra two Tac BOFF powers (one Ensign, One LtCom). Its only weakness, compared to the Nebula, is that it can't reliably cap its weapons power at 125 permanently without giving up EPTS. However it can sit at around ~115 with reasonable investment into skillpoints and equipment.
In terms of damage, comparing a Nebula using EPTW1 to another Science vessel at 115 Weapons Power; the Nebula will have an extra permanent 20% damage (to all energy weapons) plus a short duration Energy damage buff granting another 10% for 5 seconds every 30 seconds (with slotting/skillpoints).
Also worthy of consideration is that (with my usual investment into skillpoints) EPTW1 grants just under 25 Weapons Power, meaning that whilst an Orb Weaver will need to sit at 115/100 Weapons Power, the Nebula can sit at 125/85 and divert the extra surplus 15 energy points into Shields/Aux. Aux power, obviously, being desirable for a SciShip, particularly for heals, or powers like Gravity Well which use AUX to enhance their DPS.
Realistically the Weaver's extra Tac Console combined with its additional two Tac BOFF powers should push it out in front of the Nebula, but not by a huge amount (especially if you're not an Engineering captain, and thus have your own damage buffing powers to lessen the importance of Tac BOFF buffs).
Conversely, the Fleet Nebula will be able to take more of a beating with better Hull/Shield modifiers, an extra Engineering Console Slot and two copies of EPTS2 rather than EPTS1+EPTS2. Plus the extra surplus Weapons power being diverted to Aux for better heals from HE/Aux2SIF/TSS/etc.
Basically the Weaver gives up a little extra survivablity for a little extra damage potential.