Probably it would be even better for you to dump both the above families and jump into the ARM bandwagon, I think the ARM family is going to be THE microcontroller of the near future, instead of wasting your time learning old technologies from the 1980's.
Were I to advise I might say "go for the ARM" as well; HOWEVER, while the '51 may be "old technology" it is applied in more and more new places. Just think "which architecture did Burr-Brown choose for their 24bit ADC with uC?" "which architecture did IR choose for their motor controllers with uC?" "which architecture did ... choose for their .... with uC?" the answer is, in all cases, the '51.

I would say that the decision ARM/'51 (I would be steadfast on it being one of them) should be based on which type of apps the newbie wanted to make a career with. The ARM tend to be included in "medium embedded" and "large embedded" where I believe the '51 will dominate "small embedded" for years to come.

Now to the OP: throw the darn thing out, working with a '80s steam driven derivative to save $2 makes no sense.

Erik