At present stage Drupal 7 looks very mature, BUT is see some modules are not yet moved to Drupal 7 or not production Ready. For now I had troubles finding UC_Product_Import and UC_AJAX_CART. I was pretty confident that 7 was the right choice, but I am getting afraid I will need more modules that yet did not do the move to Drupal 7. What do you think?
What's your reason for looking at Ubercart in D7? I would recommend taking a look at Drupal Commerce. It's not the perfect fit for everyone and is very developer orientated at the moment (this will get better eventually). It is where the future for e-commerce is for Drupal so I would defiantly take a look at it.
If DC isn't an option then it's just a matter of looking which modules you require have a stable release and which are close to a stable release.
Another thing to bear in mind when choosing between Drupal 6 and Drupal 7 is that Drupal 6 will have support (e.g. security releases) dropped for it eventually, so going with Drupal 7 may offer more longevity.