First off, do we know yet what the Model 3 offers in terms of audio quality and interface? I mean, we're only looking at pre-production models now, and I couldn't find these specifics yet on the Tesla website.
Not that I'm trying to dissuade you from a Gen 1 Volt -- it's the best value going. But we are talking about years-old technology in terms of phone integration--with the Gen 2 Volt (and likely most newer cars) offering a more seamless experience.
With that said, know the limitations. In addition to the generational differences noted, Bluetooth streaming on Gen 1 has some issues--when running NAV (at least in a '13), you'll get an occasional "skip" or "dip" in audio at certain times (generally whenever calculating/recalculating/showing turns, etc.). It's due solely to the processor's slow performance while calculating NAV and simultaneously streaming Bluetooth. This does NOT occur when running through USB. However, it drives me crazy so I use USB almost exclusively.
Besides the skipping, I notice a marked improvement on USB audio quality--but only on actual high-quality (uncompressed) audio. Many music services compress their audio already to low-quality bitrate MP3s. Most would be hard-pressed to notice, but on dynamic music it is obvious. Honestly, it's not worth the trouble to "plug in" if the source material is already compressed/low-quality.
Also, plugging in the USB forces the unit to switch to the USB source whenever plugged in and immediately starts playing the first (usually same) song that is loaded into Apple Music (or whatever default music source). I am very tired of always hearing that same song... For whatever reason, the USB occasionally causes my iPhone to crash.
On the plus side, you can (mostly) navigate your stored music on the built-in screen so you don't have to mess with your phone when you use USB. My only issue is that it is a low resolution screen and only displays a part of music, which doesn't help much when scrolling through podcasts as they are usually episodes with the actual show number at the end (and therefore invisible on the menu).
I would STRONGLY recommend bringing your phone to a dealer and plugging it in to get a sense. Use the bluetooth, too.
Since this is a "dealbreaker" issue, definitely spend some time with it and decide if you can accept some of the limits. I think the '15 probably has the most "updated" of the "outdated" MyLink system, so start there if you can find one reasonably-priced.