Bottom line is whether you want heart-rate tracking. FR230 supports connection with a chest strap, whilst the FR235 has an optical heart-rate sensor. Purists would say that a measuring heart-rate with a chest strap is more accurate and reliable. Certainly that's the case. However, that means putting on the chest strap every time that you run. With the FR235, it simply sits and measures your heart-rate all the time, and so it measures your resting heart-rate (overnight, for example) as well as your HR during your run.
Both were released at the same time. Reports say that the FR230 has a better battery life, but I get over one week with my FR235 and I reckon that's very good.
I have a 235 and I love it, I'm afraid I can't add any useful information regarding comparison with the 230 but I do find the heart rate helpful, especially comparing weekly park runs, the fitter I am the lower it is.
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