Which Components to Upgrade

[Kalidurga]

9-speed components are getting hard to find...especially shifters. If you decide to upgrade to 10-speed, you might want to compare the costs of upgrading your present bike to just getting a complete new bike with 10-speed.

I would upgrade the shifters, rear derailleur, and cassette.

I'm finally in the position to make some changes to my bike and this is exactly how I'm planning to start. I'm leaning towards a 10-speed 12/27 because the 9-speed 12/27 doesn't have a gear ratio close to the one I currently use the most on my 8-speed 12/25. Aside from the comment about 9-speed components becoming harder to find, are there any other pros/cons either way? Since 10-speed chains are thinner, are they more susceptible to breakage if one tends to occasionally shift like a drunk monkey?

And a question about rear derailleurs: How do I interpret the Maximum Chainring Difference and Total Tooth Capacity to know whether I need a short-cage or long-cage to pair with my 48/34 compact up front? Can someone explain what this means and how it should be taken into account?