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I'd say successor, only because of the fact the vast majority (75% or so) of its games were exclusive to GBC.
Backwards compatibility with the GB is irrelevant to whether it's a successor or not, not sure why some people are using that to justify it was an upgrade.
The PS2 was fully backwards compatible with the PS1, but no one would argue that the PS2 was merely an upgrade to the PS1 and therefore we should combine their sales.
To me it comes down to forward compatibility, can the old system play the new games? If the vast majority of games can't then it's a successor, if the vast majority can then it's an upgrade. It's not enough that some games can run on both systems, that just implies that the game as been programmed to run on both systems.
In the GBC's case, all forward compatible games (GBC games that can run on the GB) were released after the GBC was released, implying that forwards compatibility had to be programmed at the game level rather than it existing at the system level. Therefore it was a successor, with some games shipping with a backported version packaged together on the GBC carts.
