With all due respects to C and Assembly programmers, I totally disagree with the advice above for non expert programmers, and expert programmers don't need our advice.
There are several Pascal compilers available for the 51 and they can accommodate any new variant. This is certainly true of the KSC System51 compiler which I have been using.
As for the merits of Pascal versus C, I think it is well explained at : http://pascal-central.com/ppl/index.html#Table
I would highly recommend a visit to this site for anyone who thinks Pascal is outdated and/or can't 'hack it'.
I would not recommend assembly language for any serious projects beyond simple timing and switching applications. It is my opinion that assembly should be limited to time critical applications, and high level language cross compilers allow the insertion of 'hand' assembled code if necessary. Unless you know, and are very efficient at manipulating the whole instruction set, I would NOT recommend Assembly. Just imagine having to multiply 2 real numbers in your application. Would you write your own floating point routine, let alone things like trig functions etc?
Code storage in no longer a limitation, and there are plenty of 51 variants with suitable flash, ram and EEprom for most applications.
From experience, a Pascal application with over 4,500 lines compiled into 62KB of a 64KB system with another 64KB used a data storage. I can not imagine de-bugging, let alone writing this code in Assembly.
I stress that this my view-point having started in this field by HAND assembling code, then using an assembler and later high level language cross-compilers.

Regards,
Tony Gelonese