eTap is certainly easier to install. If you get the Wi Fli version you can cover cassettes upto 11-36 with the RD unmodified, or as noted the Shimano HG 800
cassette is an 11 spd made to fit on 10spd freehubs, but available only in 11-34. SRAM rates the Wi Fli RD for upto 11-32, but posters have shown bikes
with 11-36 cassettes working fine with the Wi Fli RD. With Di2, if your bike accommodates internal wiring you may have to futz around with grommets for the wires,
although some frame makers have kits that adapt from cable to Di2 wire available. The etap conversion kit is just brifters, RD/FD and battery charger which
absent any wires is really easy to install by comparison.