Hi,
I use:
Jeti/Hacker Master Mezon ESC with BEC to supply servos+receiver with power and Scorpion Backup Guard (2 cell LiPo with curcuit that will let the Backup Guard kick in if voltage drop to 5 volt or below from BEC (or if BEC fail). Backup Guard weight is 38 grams.
I use this system on three F3A planes and have used this backup solution for total of 3 years without a single problem (that I have noticed).
It is a KISS solution for me, Keep It Simple Stupid.
/Bo