The 18-50mm is actually a pretty decent lens and well worth the extra hundred notes.
The key thing to remember is that the 300D has a 35mm efl of 1.6x This means that the 18-50mm is actually nearer to 29-80mm where your 28-105mm lens is more like 45-168mm.
If you want to take wide angle shots then the 18-50mm would be an excellent choice. If you want to take Macro shots you will ideally need to buy either some close up filters, extension tubes or a macro lens as neither of the two lenses you've mentioned have any real close focusing capabilities.
Close up filters are probably the cheapest way to explore Macro photography but you need to check that they work on your lenses first (some close up filters are very poorly made and can give unusable results on some zoom lenses).
If you really want to get into Macro photography then you will need a dedicated Macro lens or better still a set of bellows (but learn to walk before you run ;-) ) Macro lenses come in a range of sizes and prices too. Typically the cheapest macro lenses offer a 1:2 ratio which means that they can reproduce images at half life size. In reality these are not true Macro lenses but Close Up lenses. True Macro lenses start at 1:1 ration (i.e. they capture at life size) however the higher up the magnifcation scale you go 2:1, 5:1 the more and more expensive the lenses get running into thousands of pounds.
As a ballpark figure you can pick up a 1:1 lens starting at around £280, cheaper 2nd hand