The Vanguard ETF which matches IJR (S&P 600 Small-Cap) is VIOO, which has the same expense ratio; these two should be comparable. I would probably prefer VIOO for a long-term taxable holding, as there is more risk that iShares will raise the expense ratio on IJR in the future. Despite the low volume of VIOO, it normally trades at a small spread (3 cents according to Vanguard's data), because the two ETFs track the same index, allowing traders to arbitrage any difference.
VB (FTSE Small-Cap) is slightly less expensive but does not have as low a cap range, and with a higher yield, the expense difference may outweigh the cost savings.
And my own choice would be Vanguard Tax-Managed Small-Cap, which also tracks the S&P 600. Currently, this has lower expenses than the ETFs, and has 100% qualified dividends, versus about 75% for the ETFs.