エリックsan, Martin-san,
wow, good point about "cannot do" expressions!
It's difficult to say about "difference" even because "sotsugyou suru" cannot really take "potential" form
to compare. Basically there's no strict guideline or rules about difference in meanings when you
use potential form (affirmative or negative) and "koto ga dekiru/dekinai".
In some cases, "koto ga dekiru/dekinai" are more used in wirting and/or fomal speech.
Also, there are some cases like Eric-san wrote:
enjin oiru ha, nomu koto ga dekinai yo. You can't drink engine oil! - kyakkanteki -
That said, "jugyou ni denai to sotsugyou dekinai" would be much more commonly used than
"jugyou ni denai to sotsugyou suru koto ga dekinai" in conversation.
If this appears in writings where curriculums are explained, for example,
"....ni mitanai (満たない) baai , sotsugyou suru koto ga dekimasen".
It has more to do with verbs' meanings than "koto ga dekiru" and potential form (or "...dekiru")
Natsuko(奈津子),
Team JapanesePod101.com