I feel dumb, but SCSI vs SATA, Adding more storage?

I just bought my first server on the cheap so I could have a practice/experiment tool to work with. I always learn better hands on. First up SCSI vs SATA... So in this day and age are there still any advantages to working with SCSI instead of an enterprise grade SATA?

Example, my used Poweredge 1850 ($50 bucks baby!) came with two 147GB SCSI and an LSI hardware RAID controller w/battery backup. They are 10k rpm and Compaq branded. Suppose I had some higher capacity WD Red's and a nice RAID card for it. Which would you choose for what kinds of work and why?

How would you scale up for more storage in a system like this? This single 1U rack only houses SCSI x2 3.5's natively. Would you add an expander and run them out the back to another server or would you just add more network attached storage and why? Do the kinds of workloads make a difference?

Sorry I'm not trying to be a pain, I'm just trying to learn. Any articles you would like to recommend for study? I've done all kinds of home stuff (so I have a good background), but enterprise grade situations are totally new for me.