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The money spent could also be used for flying 
I like the 1.3x crop over FF & 1.6x, it's more of a compromise with a larger sensor and better DOF control over the 1.6 and the problem of hideous vignetting on the FF. Not to forget the benefits of lugging around 1 main body instead of 2 main bodies.
I use the 4L mainly for spotting as it's pretty light and handy. Sold off my tripod ages ago as it was too heavy to lug around (think it was a manfrotto 55CL). IS does help but it's only effective for 2-3 stops so having good light is always necessary.
I like the 1.3x crop over FF & 1.6x, it's more of a compromise with a larger sensor and better DOF control over the 1.6 and the problem of hideous vignetting on the FF. Not to forget the benefits of lugging around 1 main body instead of 2 main bodies.
I use the 4L mainly for spotting as it's pretty light and handy. Sold off my tripod ages ago as it was too heavy to lug around (think it was a manfrotto 55CL). IS does help but it's only effective for 2-3 stops so having good light is always necessary.