I think you have all the choices listed.
There are advantages and disadvantages to all:
The FXO modules for the TDM400P are the best sounding (Over the X100p in my opinion) and seem to offer the best control at the system level. Of course they cost more, but you get what you pay for. Overall, if you just want to see what you can do with the system, an X100P is probably good for a first look and development, but you should buy a TDM400P with an FXO for a production system.
Sipura ATA’s will work, but you are limited by the fact that they are really a system apart from the Asterisk box. The Asterisk system cannot directly control them, so things like being able to do a hookswitch flash to get a call waiting call are considerably more difficult. There are other limitations too, such as signalling the Asterisk box without answering the line. ATA’s will always try to answer the line and return an electronic “ring” to the caller until the call is answered by the Asterisk extension. (there are tricks to get around this though)
They do, however, offer the ability to use something other than a system with a PCI 2.0 slot in them as your Asterisk server. (Say, a laptop PC, which has no slots at all.) So they may end up being your choice anyway.
I suppose it depends on your budget, what you have available to build an Asterisk development system, and what you plan to do with the system.