Actually, shimano shifters are not serviceable (not really...i mean, you can try to do it yourself but most shops won't touch them b/c you can't get replacement parts or send them to shimano for repair--they must be replaced).

Compared to your low end shimano components, you really can't go wrong if your choices now are DA or sram. Both are great. And the shape of new DA is similar to sram (so good for small hands). Red shifters are adjustable though if your hands are on the smaller side.

The biggest difference is the throw. However, DA (new and old) doesn't take as much leverage to throw the big lever than low level shimano. You do still need to move the large lever, which isn't an issue with sram. One thing some people like about sram is the ability to shift to larger cogs 2 at a time (by holding down the lever). One thing I like about shimano is the ability to shift to smaller cogs 2 at a time (takes 2 clicks but don't have to pedal in between). This allows me to stop in an easy gear but get up to speed quickly once I get going again b/c stopping in a heavy gear is difficult for me for a few reasons (I have a bad ankle).