I have used both the TDM400 cards with FXS and FXO modules and Sipura 2000/3000 series for connection of phones to the Asterisk system.
What I noticed is that when using SIP service providers the quality of the calls was fairly bad for the incoming voice, when using the TDM cards.
I isolated that to the fact that the SIP communications in Asterisk do not have a jitter buffer (it is being implemented in the CVS version but not yet in the stable version).
So I ended up using several Sipura 2000 boxes (which have an internal jitter buffer) and my call quality has been perfect since then.
I use the G711 codec between the * and Sipuras as it is on Lan. Although that means extra transcoding for the processor as I use G729 with the SIP providers, it gives a clearer voice quality.
The other advantage for me is that when the jitter buffer will be implemented in *, I will then be able to use the Sipura boxes for the remote offices (remote workers), and revert to using the TDM card in the office.
I have the * box equiped with two network cards, one for the internet communication and one for the Sipura boxes network. There are specific setups that required in that case in the Sipura box (that I can give you if you use that setup), but it has been very successful and scalable (you just add more Sipura boxes) for us.
Hope this helps,