Both are correct -- and neither is correct.
First, the "neither" part: The MBR limit is 2^32 sectors. Given the common 512-byte sector sizes, the limit is 512 * 2^32 bytes, which works out to 2 TiB. Note that's TiB, not TB. See here, among other places, for information on the different between IEEE-1541 units (such as TiB) and SI units (such as TB). At the level of TB or TiB, the difference is about 10%, which is enough to cause a lot of confusion. Also, to add another pedantic note, one comment refers to "Tb," which is technically terabits, not terabytes -- that's off by a factor of 8; but I digress....
Worse, some disks have sectors that are not 512 bytes in size. Most commonly, some disks (mostly external USB drives, but also some internal disks) have 4096-byte sectors. On such disks, the MBR limit is raised to 16 TiB. Note that the sector size here is the logical sector size. Most modern hard disks have 4096-byte physical sectors, but since disk I/O and data structures, including partition tables, are handled in terms of logical sectors, it's the logical sector size that's important for this discussion. In any event, the upshot of this is that you might well have a 3 TB, 4 TB, or larger external USB disk that uses MBR and it will be fine.
Second, the "both" part: MBR's data structures are a bit of a mess, but the important part for this discussion is the way partitions are described in logical block address (LBA) form. This is as a start point and a size in sectors, both expressed as 32-bit sector values (hence the 2^32-sector limit). Because of this fact, an MBR disk's maximum partition size is 2^32 sectors (2 TiB, given a 512-byte sector size). This limit can't be overcome except by increasing the sector size or changing the partition table type. Because the partition start point is also a 32-bit value, it's theoretically possible to have 1-3 partitions that reside entirely in the first 2^32 sectors of the disk and then have a final partition of up to 2^32 sectors that starts just before the 2^32nd sector of the disk. Such a configuration would theoretically support disks of up to just under 4 TiB, given a 512-byte sector size. Of course, you'd still be limited to a 2^32-sector partition size, and the placement of the partitions would be extremely important. You couldn't have four 1 TiB partitions on such a disk, for instance. Thus, even this theoretical support for large disks is very limited.
Note that I've emphasized the word "theoretically" several times in the preceding paragraph. The reason is that many OSes and MBR-manipulation tools will flake out once the disk size exceeds 2^32 sectors, since these OSes and tools must still refer to data beyond the 2^32-sector point. Thus, it's not safe to create an MBR data structure in which a partition spans the 2^32-sector point, even though it's possible to create such a data structure. In fact, I ran some tests on this issue (see here for a summary of my results), and I found that the only OSes that worked with an MBR disk with a partition that spans the 2^32-sector value also worked well with GPT. Since GPT is so much more capable than MBR, there's little or no reason to risk using MBR in these situations. Furthermore, my tests did not cover every possible tool; even if you use an OS, like Linux or Windows 7, that can handle a larger-than-normal MBR disk, it's conceivable that you'll run a disk utility that will flake out on such a disk, which might then result in serious data loss.
In sum, you can use larger-than-2TiB disks with MBR if the disk has 4096-byte logical sectors. The most common way to do this is to put it in certain external USB enclosures. Using a larger-than-2TiB disk with 512-byte logical sectors, though, can be safely done only with GPT (or by not using the disk beyond the 2 TiB limit). Most modern OSes support GPT, so this isn't a hardship for most people.