I do understand the reason for wanting to minimise the number of hotels, but keep in mind that the drive between Arenal and Monteverde, although very, very beautiful, is also very long and tiring. To be precise, the last ~40 kilometres closest to Monteverde take about 2 hours on an extremely bad road and driving that road in the darkness (especially when raining) is not enjoyable at all. Believe me, my wife was very close to a stroke/heart attack. Thus, unless you leave very early for your day-tour you'll be left with hardly any time to see anything/do any activities at the "other" place.

Still, if you still decide for that, stay at the Arenal area and visit Monteverde as the latter has fewer things to offer than the former and is therefore easier 'done" in a few hours.