commuting 9 miles a day to work + around town,rear bag help

[banana seat Brian]

what I have upgraded- new cane creek SRC-5 levers. used side pull calipers and new pads, seat, handlebar, tires, chain, and other odds and ends...O and it rides like a dream

my question is I ride around 9 miles a day to work and back and around town. at this time I am not bringing much with me but am working on a rear bag.
I have a rear rack on it now.. whats a better spot for a bag on the side or on the top of the rack?

http://www.bargainoutfitters.com/net/cb/flambeau-tackle-stations.aspx?a=386264
it would fit prefect on the top or side the I only have one and the bag runs around 60 bucks so don't know if I wanna run one bag on one side at this time to keep it balanced I might try to find a matching on on ebay but for now one will be,

so my question bag on the rear top or side and why?

[Ace]

Depends of the combined weight of bag + load. You want to keep your centre of gravity as low as possible I assume, I would go for the side. Which side depends on the balance when your bike is on its stand. Hope this helps?