Max the U-Wing before using the Cantina shop to farm shard shop currency. Then only use the U-Wing to get shard shop currency from the cantina store.
As for pacing other units, use your own discretion based on gearing needs. You'll probably get everything from the cantina shop for a lark in time, but throttle up and down on what portion of your cantina cred goes to the shard shop.
I will say that both U-Wings are great ships, Bistan is a good pilot who's worth starring, and Fives is probably going to become much more relevant sooner rather than later.