Should I (a web designer) stick with gimp, or buy Photoshop?

The “TL;DR” version:

• If you have extra room in your budget, it never hurts to learn another tool.
• In the absence of an immediate need that Gimp isn’t meeting, there is no inherent “learn Photoshop” rule that you must adhere to.
• If your Gimp workflow is efficient and produces good art, what does it matter if there are a bunch of Photoshop tutorials or downloadable brushes available?
• Don’t ever give more prominence to the tools used than to your own abilities.

The verbose thoughts:

It all comes down to your needs and your particular skillset. As a many-year use of The Gimp, no doubt you are familiar with its peculiarities and interface. This is typically the largest barrier to using it – users who have been conditioned to the Photoshop-style interface – and you are already over the hump. Don’t forget that most of the people who say Photoshop is “easier” or “more intuitive” likely started with Photoshop and are conditioned to work that way. Much like switching from Windows to Mac or vice-versa, your mind has been trained a certain way and there is a cost to re-training.

There are 4 main things that Photoshop offers:

1. Tutorials

   Since you’re experienced with the Gimp and design work, this might be a wash. Many more tutorials exist for Photoshop, but I would imagine you may be able to translate them to a Gimp workflow without much effort. This really depends on whether you can read through a PS tutorial and say “ok, instead of X layer-style, I need to add a gradient and some blur and a blending layer to achieve the same effect”.

2. Brushes/Shapes Libraries

   There are a bunch of freely available libraries for the Adobe suite, but I personally almost never use them in Photoshop. Illustrator, yes – but Photoshop, not really.

3. Plugins/Filters

   I can’t speak with authority on this since I’ve never downloaded/bought/used any custom plugins or filters for Photoshop. Most of them seem to be glorified Layer Styles or Actions that come with a price-tag. If you can discern the effects being used, you may be able to create comparable scripts in Gimp that do the same thing.

4. Teamwork

   This isn’t a direct offering from Adobe, but a side-effect of the ecosystem. It’s worth considering whether or not you’ll ever work as part of a team. If you work in Gimp and everyone else works in Adobe, you undoubtedly will have trouble passing files around and sharing hints & tips.