The RT-AC68R also features dual USB ports for file, printers and 3G/4G modems sharing and a USB 3.0 port for up to ten times faster data transfers than USB 2.0.
ASUS AiRadar intelligently strengthens wireless connectivity using beamforming technology, high power amplification, and exclusive ASUS RF fine-tuning. This ensures extended coverage, dynamic data speed increases, and enhanced stability.
The RT-AC68U and RT-AC68R are the exact same router, with the only difference that the "U" stands for "universal" (e.g. can be sold by any online retailer), while the "R" stands for "reseller" (e.g. can be sold through reseller stores like Best Buy). The RT-AC68W is a white variant, the RT-AC68P is a newer hardware revision introduced in Q1 of 2015.
The RT-AC68 (P/R/U/W) V2 revision has a faster CPU - it uses 1 GHz Broadcom BCM4709 instead of the 800 MHz Broadcom BCM4708. It is denoted on the back on a sticker similar to this:
FCC ID: MSQ-RTAC68UV2 (Note the last two characters should be the revision)