I can't see the point in getting any bottle less than 12 litre in size, unless you live next door to the dive centre / gun shop etc, you will spend more time travelling to get the swine filled. For me it's around an hour and half round trip. I have better things to do than sit in traffic.
If you can't afford a larger bottle at the moment, keep pumping and save until you can. You won't regret it.
Keep an eye on the evilbay and local dive centre classifieds. You may pick up a bargain. Ok a dive cylinder has a shorter test date but a valve replacement inc test is only £60 which will convert it