The image at, say, 1:1 will be the same size on either film or sensor no matter what format is in use. Of course, you can't 'see' the image on the sensor to measure as you can with a negative/slide.
A 35mm frame is 36mm wide whereas the sensor on the Canon 300D is 22.5mm wide therefore something of, say, 30mm width photographed at the 1:1 setting will fit nicely on the film but have a bit cut off from each end on the sensor! The most popular (live) subjects in macro tend to be butterflies and dragonflies, etc., and in practice you won't use the very closest settings very often.
Very occasionally you'll get a co-operative damselfly (they're usually very flighty!) and in these instances you can go in very close with a shorter-length macro lens onto something which is about the size of a matchstick! Otherwise, the longer focal lengths give you the dramatic close-ups from a bit further away. However, there's not
that much difference in distance from subject to front of lens when comparing different groups of focal length with the nearest group, i.e 50/60mm v 90/105mm v 150mm v 180/200mm.
There's no doubt that the 90/105mm lenses from makers such as Sigma and Tamron are very popular because of the quality available at a reasonable price and whether you want to pay more for the camera makers' own offerings is up to the individual to decide.
Although macro lenses may prove better at the closest distances on the test bench, they're usually good enough over the whole range for most purposes - these two shots, for instance, were taken with a Sigma 50mm macro lens:
http://www.worldphotographyforum.com...=500&ppuser=43 and
http://www.worldphotographyforum.com...=500&ppuser=43