I meant that you won't get the full 300mb/s or 600mb/s advantage from an SSD on a USB 2.0 in bmw which caps the speed at 30mb/s and an SSD is usually twice the cost per Gb than a flash drive.
If money is not a concern than of course SSD is a better option since it can hold more Gb than a flash drive and get faster transfer to/from a PC.
About the aux port, i still believe it's a worse means of audio transmission than usb because of interference. SPDIF, Optical or USB are the same in this regard since it transmits audio in 1's and 0's.
About the Ibasso, i'll leave it at that
because i didn't really have my hands on one of those.
From my previous experience, with my limited budget and good results wanted, the priority goes to:
Speakers > Amp > Source. Because of my limited budget i'd probably never get my hands a high end Source.