What is the difference between Cash accounting and Invoice accounting for VAT?

It depends.

Cash accounting schemes – advantages for small/simple businesses:

1. You only account for VAT on sales in the VAT quarter in which you are paid (rather than the VAT quarter you are invoiced)  – because you pay your VAT to HMRC a bit later;
2. If you have simple bookkeeping records and cash expenses it is easy to extract the figures for your VAT returns without needing to keep separate sales/purchase ledger records.
3. If a customer fails to pay, you don’t have to pay VAT on the sale to HMRC (on an invoice basis you have to pay VAT on sales – even if you never get paid – and recover the VAT from HMRC only once the debt is over 6 months late).

Invoice based VAT accounting – advantages for this accounting method:

1. If your customers pay you quickly, but you are given credit by your suppliers, then there can be a cash flow advantage to using the invoice basis for VAT.
2. If you already run a Sales Ledger and a Purchase Ledger through your bookkeeping system, then it is not really an extra complication to calculate VAT on the invoice basis.