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I got a concept II cheap(er) after it had been used at the indoor championships - they get brand new ones, use them once on the day then can't sell them at the normal price so you can get a new rower (relatively) cheap. Check out the concept II webiste for details of rowing events.
I'd say a rower will condition you more than a cross trainer, as it's more of a whole body workout, but beware of using good technique - I've seen a lot of back probs caused by poor form. Again the website's good for technique.
I'm a rower who runs so I'm biased, but I would say get a concept II, but with a couple of additional points:
1. they're quite expensive so consider getting a second hand one from ebay - they're near as damnit indestructible so an older model will make no difference to you whatsoever. I bought one from ebay when I was nowhere near a river and sold it a year later, and only lost 20 quid
2. Concept 2 themselves do offer a hire service so you could always rent one for a few months and see if you need it
If you do get one, you'll find that it provides you with a great workout for your backside, quads and core stability, providing you use it properly. At the risk of being patronising, I would direct you to the following youtube demonstrations:
Some good advice here already. I've trained extensively on both (hundreds of sessions on each). Both can provide a seriously good workout, and there can be little doubt that cross-trainers are more closely "related" to running. This is why Paula Radcliffe (and others) use them when injured. But for weight loss and general fitness, I would go for a Concept 2 rower. Yes, they are expensive, but buy carefully from eBay, try to get the latest model (E?), and you'll find them cheaper than a good quality cross-trainer (I use the LifeFitness ones at my gym). Have a few sessions on both, trying the different "effort" settings, and see what you like. Decide what you want to achieve, the rower will work your upper body more than the X-T and will probably burn calories quicker at equal effort. Concept2s may be near indestructible, but I've seem them put out of use at my gym. After significant use (and not enough maintenance), the ride is much less smooth which spoils the session, so buy the best you can.
I'm probably biased towards the Concept 2 rower. I lost 4 stone in 2 months on it (16 to 12 stone) and it's still off 4 years later. I've exercised to exhaustion countless times on the rower, treadmill and X-Trainer, and nothing feels as punishing as the rower, if you choose to exercise to your limits. Although I'm currently on the X-T most days, and always have targets and aims, nothing motivated me as much as the rower to beat my times at various distances. The Concept2 website www.concept2.co.uk is well worth a look, and there are many "ranking" lists there if you want to compete against others.
Hope this is of some help.
It depends who is giving you advice, i was lucky enough to be at Liz McColgans fitess suite in Scotland. I told her i had a rower, her answer was get rid of it and replace it with a stationary bike.
She maintained for a runner the bike was the best piece of equipment and also advised that you should get to a point where you are standing up on the pedals.