There is compatibility mode for 32 bit so there is not much point in caring unless you need tons of memory(which is pro).
There's a compatibility mode for "everything", I don't think that's a valid reason to not do something "new". Only pros need much memory? Why?
That you can use VS Express to indeed build 64 bit binaries has been said already. Other than that, you can get a trial version of VS which is able to build 64 bit binaries. Students get the Pro editions for free. Other compilers support 64 bit binaries out of the box. Clicking on "64 bit" is definitely not what only "pros" are capable of.
if you buy for few hundred dollars - you're a pro
I don't see how buying something makes you a pro...
Other compilers are for pros[...]
Why?
It requires 2 builds if you want to support both (unless you abandon like 15-30% of user base and go only 64 bit), therefore you're a pro for knowing how to configure it all and ship two versions.
It's what a developer does?! Maybe it's harder for "non-pros" to build 32 bit stuff because they find it logical to choose 64 bit because their operating system says so?
I don't know what you are trying to say with your pointer size nonsense or registers and calling conventions. We are talking about compiling for a target architecture, which often is 1 configuration setting (like a checkbox in SFML Installer), not building an assembler.
Personally I think 32/64 options make sense in a program like SFML Installer, just like you have static, dynamic, release and debug options.