I have used a variety of these products and will give you my 2 cents.
What advantages are there getting a firehawk fx rather then an amplifi fx100, amplifi TT, amplifi 150 or amplifi 75?
There are three main differences between the Firehawk and the Amplifi group of products. Firstly, the Firehawk uses HD versions of some of the amps and effects. The HD versions of the amps and effects are of a higher quality than the standard versions. The HD versions are the same amps and effects built into the POD HD series of multi-effect pedals. Aside from the HD amps and effects, the Firehawk and Amplifi share all the other amps and effects and in fact any patch created for the Amplifi will work with the Firehawk. The same can't be said for Firehawk patches. The second major difference between the Firehawk and the Amplifi is the Firehawk has a built-in port for the Variax guitar. By plugging a Variax into this port Firehawk patches can control the settings on the guitar as well as to provide power. I have a Variax and I love it. To me this is a very important feature but if you don't have a Variax or have no intention of purchasing one it won't be important for you. The third major difference, and in my opinion a killer feature, is the built in effects loop. With this effects loop you can add any effects pedal you want and you can control where in the effects chain you put the external pedal.
Both the Firehawk and Amplifi series of products use your iOS or Android phone or tablet to edit patches. Its simple to edit patches and there are thousands of patches to choose from.
What are the advantages of the Pod HD500X over anything else?
***I had a Pod HD500, before it grew legs and walked away from a jam session I was at. The only major differences between the two units is the 500X has slightly more processing power than the 500, and the 500X has better footswitches.***
The Pod series is Line 6's professional series of multi-effects pedals. It's a well built and feature rich multi-effects pedal. You can dial in a very good sounding tone if you take the time to learn the patch editing software. It installs on your PC or Mac. It takes more effort to create a patch than the above units because there are so many parameters you can edit. Many people find the sheer number of parameters overwhelming and quickly become frustrated. You can find plenty of ready made patches for the unit but not nearly as many as the Firehawk of Amplifi.
The 500 series also has a Variax port and an effects loop. You can also purchase additional amp packs for the 500 series multi-effect pedals. I suspect these packs will end up being offered to Firehawk users but for now this is a 500 only feature. The one feature the 500 series has that the Firehawk is missing is the ability to create multiple Set Lists.
*When I had my HD500, I purchased a Windows tablet so I could edit patches on the fly. It simplified the process a little and if you end up going this route you should pick yourself a cheap Windows tablet*
Also what is better about the Pod HD Pro compared to the HD500X?
The Pod HD Pro is basically a rack mounted HD500X.
So after all my rambling, my recommendation is solely based on the features you require.
If you want something easy to program, don't need the Set Lists and don't have or never intend in getting a Variax, an Amplifi product is likely sufficient for your needs.
If you want something that's easy to program, has some of the most realistic effects Line 6 has to offer, have or are looking to get a Variax guitar, the Firehawk is the way to go.
If you require deep editing of patches, multiple Set Lists and want the option to purchase more amps, have or may be considering a Variax the HD500X would be the way to go.
As for what type of speaker should you get with a multi-effects pedal, you want a speaker that will not change the sound. So that means a good pair of monitor speakers. There are a few threads on this board that already address this question. I use this and it works well for me:
http://www.voxamps.com/soundboxmini
I use the Keyboard 3 setting and it works beautifully.
As for me, I love my Firehawk and aside from the Set Lists and current lack of additional amp models, I haven't looked back.
ppiluk's post in Firehawk FX vs Amplifi Range vs Pod HD X and Speakers vs Amp with multi-effects? was marked as the answer