If you like underscores as a starter theme & if you want to use Bootstrap as well… who said you can’t have both? 
Checkout _strap which is underscores + Bootstrap. So you can now get up & running on all the responsive & grid stuff straighaway. It has LESS included by default and if you’re into SASS instead then grab your Bootstrap SASS files from here - https://github.com/twbs/bootstrap-sass
If you do end up using SASS then just one more thing - there’s a _strap.less file in the theme with some custom styles specific to the theme itself. Those can be converted to SASS easily - manually or search for one of the many LESS to SASS conversion guides online.