Two seconds or five seconds, you mean? Mine are up to temp by the time I get the iron and solder in place (T245 handles). The time is also going to depend on the tip. Some have "high thermal efficiencies", some have large mass. Can't see that being much different with an induction iron, it's still just power delivery.
Come to think of it, the variety of tips available is another thing to consider. There is not going to be a big variety in tips until a series of iron has been around for a while. For example, Hakko has 10.4 and 15.7mm "spatula" tips for the FX-100 (in two temperatures, 400C and 450C). Metcal looks to have 10, 15.75, and 22mm, JBC has 10, 15, 21, 32, and 37mm. Whether it matters depends on the operator and usage of the iron. Most people will be happy with a few typical shapes and sizes, but some will find extra versatility valuable.