My understanding is that all bikes in this price range are about the same in terms of frame quality and componentry. They are all hi-ten (read "cheap and heavy") steel frames with lower quality components. I'd pick whichever one you think looks better and/or you can get a better deal on.

The great thing about bikes in this pricerange is that they're cheap, rideable and relatively good looking. However, you can get a LOT more bike for your money if you look around a little more and are willing to stretch your budget a bit.

Depending on your area you can find something used thats double the quality of one of these bikes for a similar price. Alternatively, you can sometimes find an older model of a bike one step up the price/quality ladder for a discount. My first fixed gear was an older fuji track that I bought for like $250 brand new (it has better/lighter steel tubing and slightly better components than 6ku et all and usually goes for like $400)

My advice, do some shopping around. If you cant find anything or arent willing to stretch the budget, just go with what you think looks better and plan on upgrading to a better bike down the line if you get into fixed riding.

Have fun!