I'm going to go out on a limb here and start sawing....
I think Rebels are stronger at present as they have access to better gimmick list options.
With two balanced fleets (Say 1 x large, 1 x medium, 1 x small, plus some fighters), sure, the fleet strength of the factions is pretty balanced and it should come down to game play.
But Rebels have options for completely asymmetric fleets that can't really be countered by a balanced Imperial fleet. Whereas I think a well designed rebel list can probably deal with any likely combination the imperials can field.
For example, the "MSU" MC30/CR90 lists led by any of Riekan, Monmothma or Ackbar, can all do horrendous amounts of attrition to big Imperial ships and basically control the terms of the fight, without having a single big target that is worth dedicating an ISD to destroying (even if it can catch it).
Rebels can also do things like the Riekan zombie CR90 engine techs fleet, again which a balanced Imperial build would have massive trouble dealing with. There isn't an Imperial equivalent to this.
?
Can you give us these lists?
The issue i find with the CR90 Rieekan list and the Rebel MSU lists are that they cant usually win by big margins.
CR90 swarms with Rieekan have the inherent concept that you are sacrificing ships to take down bigger ships. While fine, if you lose 1 you can kiss that 10-0 goodbye, you lose 3 you are now in the 8-2 range and so on.