Nope, no difference except for the ones stated in the plan - mostly the disk storage and bandwidth differences.
They'll be fine with it - though as is usual with most shared hosting providers, they'd prefer that you didn't use anywhere near your quota!
Also, remember that you can always upgrade to a higher level plan later. You can even upgrade if you've prepaid for 1 or 2 years. They'll apply your leftover prepayment to your new prepayment plan and you just have to pay the difference.
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