I'm not familiar with AG, but that is a nice looking bike. I don't like the frame geometry on the Kestral at all, looks more like a TT frame to me.. Shimano 105 and Sram rival are the 3rd group down the scale for both companies and either would perform great.
I'm not a Sram fan, but with those 2 choices I would go with the AG if it were me..
I just did what I'm sure you must have already done and googled AG. A quick look didn't show any reviews but I did find their Web Site. The price they list on ebay is the same price listed at their online store. Toray, the Japanese company that manufactures the tubing does check out as legitimate. And of course all the components are made by reliable companies. The price certainly looks right. The question of the quality of frame construction and wether the geometry is right for you is hard to answer. I have to agree with wales that if the kestrel has TT geometry I would pass on it. In case you haven't already been there here is the link to AG's site https://www.agbicycles.com/AG--Complete-Road-Bike-white-and-Red-Year-2012.php
I bought a 2001 Trek Full carbon fiber.
Took it to a shop and they told me what needed to be replaced.
Ordered new parts from ebay stores.
Then paid a bike shop $65.00 plus tax to replace parts.
I installed tires and tubes myself.
Now for less than $500.00 I have a 17 pound Carbon fiber bike with Ultrega parts that runs like a newbike.
Trust me a lot of my columbian junior friends have these ag bikes so I've rode them and they are amazing but both bikes really benefit mostly to racers and depending on if your a racer or not you may want to find some other choices
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