On a bike like a Sirrus, I'd go for a 48t, unless you have lots of really hellacious hills (>8%). A 48/28 combination is doing 8mph at 60rpm. Most people riding an ebike are going to want to go faster than that uphill. Meanwhile a 48x14 gear is a nice sweet spot at 22mph at 80 rpm, which is a good cruise speed and rpm for flat ground.