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Nowadays I would definately go with an Ethernet based solution such as Ethernet over Copper. It has the same SLA as a NxT1 but often a lot cheaper and native Ethernet. Plus it is easy to up- & downgrade bandwidth.
NxT1 is going to be a noticeable improvement over T1. I'm sure it varies somewhat based on who your provider is, but here we have options ranging from 3Mbps to 12Mbps, which is quite a lot faster than the 1.5Mbps you can expect with a T1 line. NxT1 is a lot less expensive than the next step up, a T3 circuit, which is usually not cost appropriate for SMBs anyway, not to mention that T3 circuits are often not even available at many locations. NxT1 uses existing T1 lines where possible, plus you don't have the asymmetrical bandwidth concerns that you mentioned with DSL. If you have outgrown a T1 line, I would definitely look at the NxT1 option.