The two meters are VERY similar. The biggest difference between the two (as you know) is that the 508 has a built-in 1 degree spot meter. You can purchase a 1 degree spot meter for the 358, but by the time you purchase the 358 and the 1 degree meter, you're approaching the cost of a 558R, and the R has a Pocket Wizard transmitter built into it. That is a $50 option on the 358. Add it all up and the price difference is miniscule. Also, the 1 degree spot meter on the 358 only measures down to EV5, you have to set it on F8 in order to get a reading. The spot meter on the 508/558 measures down to EV1, so it's much more sensitive.
Ok, so what can a 1 degree spot meter do for you? A whole lot if you know how to use it (in conjunction with the zone system). For one, you can measure very specific parts of your composition. This information can tell you where your highlights and shadows are in relation to your basic exposure. For chromes and digital you want to keep all of the important parts of your scene within a 5 stop range. So if your base flash exposure is, say, f8 but your shadows measure f4 and your highlights measure f22, something's got to give 'cause you're either gonna blow out the highlights or have mud for shadow detail.
A spot meter is also good for measuring backgrounds and making sure they're evenly lit. You can stand at the camera position and take measurements from various background locations to check it. It can help you raise or lower the background value. If you take a basic Thunder Gray background, you can over or underlight it to make it appear anywhere from pure white to pure black. For white, set the power of your background light so that the background reflects 3 stops more light than your main subject light. For black, reduce the power of your background light so that the background reflects 3 stops less light than the main. (You can actually do this with any color background, but Thunder Gray is easier to work with).
If you want to rim-light a model from behind, you can make sure that the exposure is even by measuring his/her outline. I could go on and on, but you get the idea. Once you start using one, you'll wonder how you ever got along without one. I consider it an indispensible tool.
-Chip
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