If you go with stevieb's libs, and switch to RPi:

* Working together is more fun. stevieb will get an incentive to mature his libraries (win for the community) and you will get good support.

* RPi's are more expensive. Think also about theft/raindamage/overheating of hardware and how to secure it (high pole, or inside a fake? hive or bury the hardware). Although, on second thought, I guess that bees guard the hive well. BACKUP your data!

* Graphics can also be made with javascript and because some have CDN links (which means the javascript code is downloaded from elsewhere) could potentially be lighter than generating images, as all you provide is the data with some initializing code.

I also like the fact you are going for RRD. Remember you can add data to more than 1 database, so if you collect each half an hour, and have 48 measurements, create another RRD that feeds on that first information, but summarizes the data into, say, 8 hour chunks (3 measurements per day). This way, you can have daily data, and weekly/monthly averages, easy to query and display on your webpage. And at fixed databasesizes.