The 18500's are a bit cheaper and a little shorter but they are limited to app. 15 amp draw. For me, I would take the 18650's because you can get 30-35amp and more battery life for only a slight increase in size and price. I picked up a pair of purple Efest IMR 30 amp 18650's on ebay for like $18 (free shipping).
More amps will allow you to build lower ohm coils and get much more power to your coils. Check out
Ohm's law | Steam Engine | free vaping calculators to see the difference in amp draw and watts based on the resistance of your coil and the battery's voltage (use 4.2v vs the rated 3.7v for a fully charged battery)
For an RDA I am really digging the Quasar clone (purchased on ebay for app. $14.00 + free shipping), great stock airflow, good for single and dual coil setup. Dual nano dragon coil at .5 ohms, it is a beastly fog machine. Feels like taking "water pipe" rips vs sipping vapor thu a coffee straw IMO, YMMV.
-Best of Luck-