I didn't really find anything useable on the Digitech Synth, and the straight envelope filter is lacking IMO. However if you ever want to play Technologic by Daft Punk, the EXACT sound is on that pedal lol.
I would personally go down the route of seperate Fuzz, Octave, Envelope/Wah pedals. Simply so you have an approximation of the synth sound, but you can also use them individually for any other tunes.....therefore spreading the cost across your set rather than £200 on two songs
For example:
EHX POG (I think an OC-2 is a better -1 octave, but the POG gives you +1 which helps for synth sounds)
MXR Envelope (love this filter, one of very few pedals where I appreciate the clean blend)
Fuzz (take your pick of brand)
Whether the Fuzz sits in front or behind the filter is your call.
To clarify, I've not listened to either track you've mentioned, so just going on 'generic' synth sounds here
Si