I've played both expansions and they're (to me) identical.
Both expansions have one new card-mechanic, and one on-track mechanic, which only applies to one track each from the respective expansion and both expansions introduce aggressive AIs, iirc (the chevrons).
So, I think, you should decide which card mechanic you prefer, either the cooldown from discard (Heavy rain/Supercool), or easier slipstreaming (Tunnelvision/Draft), because these will apply to all tracks.
Or you can print tracks made by the community, if you simply want more tracks, but I have no experience with those.