[Brightcolours]

 Surely 400mm on apc = 600mm on ff?

 

The fuji is a 1/3 stop faster at the long end, but FF has roughly a stop advantage so 2/3 stop advantage to the tamron/5d setup?

FF has a 1 1/4 stop (for 1.5x crop, 1.6x crop is 1 1/3rd stop) advantage over APS-C (if the sensor tech is comparable). So a 1 stop advantage of the Tamron.

DOF wise same: f2 on APS-C is equivalent to 2 x 1.5 = f3 on FF. 

1.4x is one stop (see 1.4x TC's).

 

So, the choice is 1 stop advantage or less size/weight.

 

On reach: if you look at it from a maximum magnification perspective, FF sensors are available with higher resolutions than the Fuji sensors.

 

JoJu makes a good point about the Tamron also mounting just fine on APS-C bodies from for instance Canon and Nikon.