Depends on your equipment and how you are connecting.
E&M connection - No CSU
FXO/FXS - No CSU
T1/PRI - need a CSU
I would suggest the PRI. I feel it is easier to troubleshoot. You have many options for connections. I also suggest the AS5350/AS5400 as your choice for a gateway. You can do a lot with it. If price is a factor, the MC3810 is a very cheap (see ebay.com) solution, but requires some prior planning. You can get an MC3810 that can handle 1 or 2 T1s and 24 calls for less than 400 dollars. If you want it to handle 48 calls they get expensive. I would just by two MC3810s for 2 t-1s. Gives you a bit of redundancy.
It is what it is!!
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A+, Net+, I-Net+, Certified Web Master, MCP, MCSA, MCSE, CCNA, CCDA, and few others (I got bored one day)