In my humble opinion, BTS is so much more than Smart Coins at this point in time. We shouldn't let that one feature shape our whole worldview.
Whether it's more liquid Smartcoins or an even more awesome, popular sellable product/UIA, the ability to transfer them privately will surely increase potential demand for them and as a result increase the value of BTS shares.
However without sellable product, stealth transfers won't necessarily increase the value of BTS unless the belief is that BTS is currently undervalued because potential shareholders are reluctant to buy this amazingly undervalued company without privacy.
So I would think focusing on SmartCoin or other 'better than Smartcoin' product would ideally precede Stealth, not critical though.