70-200 2.8L IS II + 1.4 iii extender vs 70-300 4-5.6L IS

I'd love to hear from shooters who have used both....

I love the compact 70-300 4-5.6L IS , which has better IS.... but if you have the 70-200 2.8L IS II + 1.4 iii extender (=f4), is there really a reason to go for the 70-300 besides compact travel size?

Thanks

Dave

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pro photo-illustration
http://www.DaveSeeley.com/
current kit: 5DmkIII, 5DmkII, 17-35/2.8L, 24-105is/4.0L, 28-70/2.8L, 70-200is/2.8L, 100 macro/2.8L, 50/1.2L, 2x II extender, 8x550ex Speedlites, Phottix Odin Flash Triggers