There is a fair amount of overlap in what they do, the main variation is in the interfaces and how the information is presented. I tend to use one or the other depending on the task at hand. They're both of good "pedigree", xdebug has been around now for about 10 years I think and so has a good amount of history behind it, and XHProf which XHGui is based on was developed by Facebook and used against their code base which is probably somewhat larger than yours (though hopefully better written!). At then end of the day they're both pretty easy to get up and running (and of course they're free), so I would suggest giving them both a test run and see which you prefer the feel of and which better suits your needs in terms of the information it gives you for your task. Given that you look like you will need all the help you can get, you might even end up using multiple tools like this to get as much insight into the code as you can. If you do, be aware that they can often interfere with each other (or so I've read, I've never tried those two on the same code base at the same time) so you might need to deploy them on separate virtualised but identical environments with the same code.

Edit: Just to say, I usually use XHGui for profiling existing code and code in production, and xdebug for profiling changes to code and code under development. But thats just because thats how the tools "feel" right to me, and there's no reason why you can't do both with both.