First, it's very likely that the hybrid Mustang and F-150 will share a hybrid powertrain. The two already use several of the same basic powertrain components, including the 5.0-liter V8 and rear-drive automatic transmissions. Supporting this theory is Ford's confirmation that the hybrid F-150 will be rear-drive. The F-150 and Mustang are also slated to get a new 10-speed automatic transmission that was developed jointly with GM. There's a very good chance the hybrids' electric motor(s) will be packaged in a version of that transmission, replacing the torque converter. Ford hasn't said what kind of engine will be assisted by electric power, but we think a four-cylinder – either turbocharged or naturally aspirated – is a safe bet. Battery placement is an open question – we expect them as low as possible, although the F-150 could house them somewhere in or under the bed.
Ford is promising power as good as its current Mustang V8 with more low-end torque. The Mustang's 5.0-liter Coyote engine puts out 435 horsepower and 400 pound-feet of torque. Of course, some additional output is likely to be canceled out by added weight from the motors and batteries, so we'd expect Mustang hybrid performance to fall somewhere between that of a Mustang equipped with the 2.3-liter EcoBoost I4 and one with a 5.0 V8.
There will definitely be differences in the way these two hybrids are set up. For example, the F-150 will have the added capability of supplying AC power by running the hybrid powertrain like a generator when it's parked. This will come in handy for people who use their trucks for real work on a job site. (And also tailgaters.) That feature suggests these won't just be mild hybrids with a small assist from a starter-generator. Fun fact: Ford and Toyota were working together on a hybrid pickup, but the plan fell through and neither side was too happy about it.
We don't expect these hybrids to do much besides add a little power and reduce fuel consumption compared to V8 versions of the truck and pony car, which will be accomplished by using the electric motor(s) to augment the output of a smaller gas engine powering the rear wheels. Other normal hybrid functions should include stopping the engine when the vehicle is stationary, recharging the battery through regenerative braking, and generally trying to mimic automatic-transmission versions of the base cars. Knowing Ford, they'll also have some sort of piped-in V8 soundtrack for those who long for those missing cylinders.
Given the announced timing – 2020 availability for the F-150 hybrid and a 2020 debut for the hybridized Mustang – both are likely to be based on the next-generation versions of their respective models. Both the current F-150 and Mustang went on sale as 2015 models, so assuming a six-year production run both will be replaced for 2020. A new Mustang could take a cue from the F-150 and use more aluminum to save weight, which would be a big help for the hybrid model. New platforms for both would also make it easier for Ford to package the batteries in a safe and efficient way.
We'll likely get answers to a lot of these questions in bits and pieces between now and 2020, and you can be sure we'll be following this interesting development very closely until then.