the first one you linked has a steel chainwheel, while the 2nd one has an alloy chainwheel. steel chainwheels might last longer, but are a bit heavier and also sometimes noisier in my experience. The alloy one has 130mm "BCD" (Bolt Center Distance), which is probably better for a fixie application, but means the smallest chainwheel you could put on it is 39T, while the 110mm spider would take a smaller chainwheel (not that you're likely to want smaller on a fixie).

those are both 170mm long arms, which is the 'standard' size. you might look on the back of your current crank arms and see if there's a number like 165, 170, 172.5 or 175 (those are the common crank arm sizes). longer arm gives you a little more leverage, but means your knees travel farther up/down. its all a tradeoff. short legs prefer shorter crankarms. shorter crank arms are easier to 'spin' (pedal at high cadence), while longer arms tend to promote 'mashing' (pedaling with high force).