Which Rebel build would be more effective? The classic 4BZ, or 2 Bs and 2 T-70 Xs with a few small upgrades such as Advanced Sensors on the Bs and R2 Astromech with Integrated Astromech on the Xs. Or perhaps Fire Control System on the Bs and Autothrusters on the Xs. Either way, XXBB definitely has more agility, but not as many red dice as 4BZ. Thoughts?

You could run 2 Advanced Sensor blues with title and tactician, along with 2 rookies with an R2 and IA.

I like the non-4BZ list will win out here. I never played as a 4BZ but I have played against them with XXAZZ and my X-wings just tore up some Bs with PS kills (Tarn/Luke) and I got some key blocks in with my proto...anyway, the point is that I feel that X-Wings are now even more superior with IA options...and the T-70 with AT is just killer.

Warning! Incoming personal opinion: The best list here is going to be a non-symmetrical build somewhere in the realm of BBXX...maybe with a Y. An example of an effective generic list would be the following:

"tool box"

24 - 1x Blue Squadron Pilot + FCS

26 - 1x Blue Squadron Novice + AT

26 - Stress Hog

24 - Tarn + R2 + IA

Tarn w/o R7? Yes...it's outside the box...but Tarn is a great fighter in his own right. This list creates targeting choices for your opponent and gives you options for large ships via stress, turrets via AT, and has some serious well rounded defense to top everything off. You adapt the play style depending on what you are facing. The sleeper is the FCS B-Wing who can start putting out some serious damage if left ignored.

To fully answer the OP and go with ONLY Bs and Xs, I would do BXXX:

24 - 1x Blue Squadron Pilot + FCS

26 - 1x Blue Squadron Novice + AT

25 - 1x Blue Squadron Novice + R2 + IA

25 - Tarn + R7 + IA

Poising for when tractor beam hits the scene...X-Wings offer a good balance of firepower, speed, and agility. Having a squad full of Bs with tractor beam out there is just asking for trouble... In fact, once it hits, dropping it on the single B in this list moving down R7 to R2 on Tarn would be ideal.