I realise this is a bit of a loaded question, but I'll throw it out there and maybe I’ll 'hear' some viewpoints that I haven't considered. I'm looking to purchase a new piece of glass for my wife (aspiring sports photographer). Primarily she wants to shoot hockey and MMA (Mixed Martial Arts) – for which she uses her 70-200mm f/2.8 L IS USM and 24-70mm f2.8 L USM respectively when she’s able to get rink/ring side.

However, there are some situations where 200mm just doesn't quite have the necessary reach (e.g. shooting hockey from an NHL arena’s balcony and 280mm with the 1.4x extender is too slow; or shooting track/field sports where even 300mm can still be a bit wide).

Originally I had planned to purchase her the 400mm f/2.8 L IS USM on the advice of another sports photographer – citing it as possibly the best sports lens money can buy, and frankly I’ve found little argument to the contrary.

I was all set to place the order, when I began to worry about its size and weight. My wife is only 165cm (5’5&rdquo and 45kg (100lbs); the 400mm f/2.8 L IS USM is over 40cm (16&rdquo long (with hood attached) and nearly 5.5kg (~12 lbs) in weight. She’s currently shooting with a 50D, but I’ll likely purchase her the 1D MkIV (or whatever Canon chooses to call it) when it’s available (provide the reviews are positive) – which could see the total assembled gear weigh-in at over 6.5kg (~14.5 lbs). She already finds her 70-200mm f/2.8 L IS USM too heavy to shoot handheld (thus she shoots exclusively with it on her monopod with no troubles) and can only manage her 24-70mm f/2.8 L USM handheld for a few minutes at a time. Obviously she’ll never be shooting with the 400mm f/2.8 L IS USM without a monopod and she’s likely not going to be moving about a whole lot with it either (i.e. she’s likely not going to be running up and down sidelines with a 400mm over her shoulder, a 70-200mm hanging off her other shoulder and a 16-35mm holstered to her hip).

That got me thinking that perhaps the 300mm f/2.8 L IS USM might be a better option for her, as it’s less then half the weight and almost half the length – albeit at the cost of 100mm in focal length.

I’d hate to spend the money on the 400mm f/2.8 L IS USM only to find that it’s far to large and heavy for her to manage, and have her leaving it at home most of time, or that it’s too tight to be useful for the main sports she wants to photograph. None of the camera stores in our area have a 400mm f/2.8 L IS USM in stock, or even available for rent, so there’s no way to ‘try one on for size’.

I'd really like to avoid any scenario that has me buying both! Frankly my wife is spoiled enough.