Which fund should I choose between HDFC small cap and SBI small cap to invest for an SIP (for a period of 10 years), given my risk appetite is very high?

Both funds have done well and outperformed the category benchmark, though SBI has a higher 5 year performance. It may be a good idea to split the SIPs into both the funds.

However do make sure of the following THREE things:

1. Your investments are in line with your risk profile. Only small cap is not a good idea even for a very aggressive investor. A mix of small/mid cap and largcap + multicap will give you better results over a long period.
2. Invest only in direct plans of these funds, as there is a considerable commission outflow in the regular plans. If you are already invested, switch them over to direct online.
3. A 10 year horizon is good to invest in such funds, so stay invested through thick and thin. Here again a good asset allocation will give you a nice springboard to go more aggressive when markets correct.