The Manta's larger footprint is completely worth it as it has two footswitchable presets. The jacks are also top mounted so, depending on which patch cables you use, the difference in footprint vs MXR BEF may not be as huge as it initially seems.
Also, IMO you can get the Manta to sound like the MXR but there are so many sounds in the Manta that you'll never have a hope of replicating with the BEF alone. As far as pure bang for your buck I have to say Source Audio really nail it. There's a reason they have such a huge thread over on Talkbass - a lot of folk over there run purely (or mostly) SA boards with the hub and a MIDI switcher to give them access to literally hundreds of tonal combinations.
Anyway, so as to avoid derailing the thread - I'm really eagerly waiting for Source Audio to release an octave pedal...but it seems that their focus is on support for the Nemesis delay and Ventris reverb, along with expanding the Neuro app and Neuro Hub (AKA Soundblox Hub, they're the same thing).
Looks like we may have quite a long wait for a SA octave/pitch pedal, much to my chagrin!