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CGI - Pros: You probably already know RPG. Easy to start with.
If you change "CGI" to "CGIDEV2" then I'll agree with that! Getting started with CGI when you aren't using CGIDEV2 (or a comparable toolkit) is not easier.
CGI - Cons: Doesn't always scale well. Locks you into using iSeries as your webserver and as your back-end. HTTP server and web application must be in same tier. Tough to hire people that know anything about RPG-CGI.
These are more the cons of RPG than they are of CGI. Most of your restrictions come from the fact that RPG really only runs on iSeries systems.
It doesn't really lock you into using the iSeries as both the web server and back-end... The web server does need to be iSeries to run RPG, but an RPG program can talk to other computers to get the data. You could have a back-end computer running DB2 and query it from RPG. Or RPG could run a web service on another computer, or communicate with another computer using data queues or sockets.
It's just less common because there's such wider support for Java in the industry. JDBC drivers are easy to find, so communicating with databases over a network from Java is much more common.
Like I said, your restrictions are really because of RPG, not CGI. The major cons of CGI itself are: a) It requires starting up a separate process outside of the web server.On most platforms this is very expensive. It'd be like submitting a new job for every RPG program you run, waiting for it to finish before doing the next step. The overhead of submitting the job is significant.
However, on the iSeries it isn't such a big problem because the CGI jobs remain active and are re-used. You don't have to start new ones over and over. Plus, iSeries programs can remain in memory with their files open and variables intact so that they can be called again very quickly. In RPG we think of this as ending with *INLR = *OFF, but it's available via activation groups in other ILE languages as well. Since no other platform has this functionality, and iSeries is such a tiny niche of the market, this is still considered a huge blemish on CGI in general.
b) CGI programs traditionally have the HTML hard-coded in them, making them hard to update.
This isn't a problem with CGIDEV2 or similar toolkits. Again, most platforms dont' have these toolkits for CGI. So the ease of encoding the HTML in a JSP or PHP script is extremely attractive. On the iSeries, we can do something very similar with CGIDEV2 or similar, so the difference isn't so big.
c) There's all of the work of decoding the variables from web forms, storing them somehow where it's easy to utilize them, etc.
Again, CGIDEV2 and similar tools do that for you, but on other platforms the tools that do this aren't very good, or do less of the work for you. For a long time, the standard on Unix systems was to code CGI in Perl using a module called CGI.pl because it helped decode these variables. It didn't help with the HTML part of it, so when PHP had both the functionality of CGI.pm and the ease of coding the HTML, it blew Perl out of the water.
Unfortunately, most people who have programmed in CGI aren't iSeries people and haven't ever seen a toolkit like CGIDEV2. To them, CGI is old and awkward and slow because of the above reasons.
JSP Pros: Easy to find people who know it. Can run web app tier on just about any platform. Can connect to multiple back-end databases. Can separate HTTP server tier from web app tier. Lots of open-source resources.
I actually haven't had much problem findint people familiar with CGIDEV2 programming. But, there are more Java developers than iSeries *users*, so it's definitely easier to find Java developers familiar with web programming. And drastically more open source resources, since 95% of the iSeries community still doesn't understand open source.
However, there's even more PHP open source out there. PHP is also extremely widely used, at least as widely used as Java.
If multiple tiers, back end databases, etc are important to you, .NET should also be given serious consideration. There seem to be more .NET developers out there than Java, as well, and aside from the open source aspect, it matches all of the other pros of Java.
Another Pro of using Java that you didn't list is that there's lots and lots of functionality available. By that, I mean routines that assist you with writing web applications. Far more tools than CGIDEV2 provides. (Though, again, .NET and PHP have most of the same tools as Java)
JSP Cons: You probably don't know it. Can be tough to get started if you don't have an experienced Java person.
Also, for a small shop, performance can be a big problem with Java. That's my biggest gripe about it.
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