Reservoir capacity question

I'm going to be doing a small closet op with 4-5 plants and was wondering if it really mattered how big the reservoir is. Reason being, im stuck between these 2:

HYDROPONICS 4 PLANT HYDRO SYSTEM + GROW LIGHT GUIDE - (eBay item 200159365241 end time Oct-10-07 21:52:15 PDT)

and

BIG 5 PLANT HYDROPONIC SYSTEM! HYDROPONICS - (eBay item 200118913440 end time Oct-09-07 16:02:12 PDT)

Whats would be the pros and cons of each one? I understand id have to be filling the smaller one up with more water often, but thats not an issue for me. If anyone has any other suggestions or experience with either of these systems, id appreciate any input!

for a 5 plant setup you ll want a 20 gallon res as the roots from ganja will fill the res fast and more room means better roots growth.Small res will make the plants fight for food and water

ah, thank you for the good info!