While both options are good ones, note that the displays are not the same.
The TI "BOOST-SHARP96" LCD BoosterPack is based on the now-obsoleted Sharp LS013B4DN04 display (96x96 pixels), whereas the Silicon Labs "EFM32 SLSTK3400A Happy Gecko Starter Kit" contains a Sharp LS013B7DH03 (128x128 pixels) which is still an Active Status component. I have a TI "BOOST-SHARP96" LCD BoosterPack with a FRAM Launchpad and have been satisfied with it. It can (apparently) be used with many different Launchpad models.
Again, both boards are potentially good options. Your personal choice may well depend on what toolchain you use, or if you are aiming to try TI's Code Composer as compared to SiLabs Simplicity Studio. At these prices, you might want to try both!