The 235 has a heart rate monitor, but is otherwise the same as the 230. It's much more expensive than the 230 at present (probably because there will never again be a GPS watch that doesn't have heart rate built in). The question is simply whether you want a heart rate monitor on your watch or not.
No optical heart rate monitor on the market is really good enough to do serious heart rate based training, if that's your thing. They're accurate enough to set a speed limit on your recovery days, which is honestly the main use case for heart rate for most runners. The newer Garmin heart rate monitors also track resting heart rate throughout the day, which is good for tracking your recovery, but the 35 doesn't really do that.
The advantage of the 230 is that it has more data fields and you can download customized workouts, which can be extremely detailed. Some people find them very useful, but others prefer to do most of their hard workouts on the track. If you do complicated marathon workouts in the course of your long runs (e.g., including 1.5 mile MP repetitions alternating with .5 mile "steady" recovery intervals), then programming your workouts is very helpful, because it's easy to zone out or lose track. It's also nice to be able to do a hard track-style workout on the roads on occasion, especially if you're a road racer. Quarter mile repeats and below aren't very accurate with GPS, so you can't stress about a few seconds, but longer stuff usually works well.
That said, the 35 is overpriced and the 230 is due for a refresh. If I were getting a watch now, there's no question I would get the Polar M430.