I personally went with Regular Account. Primarily I see p2p lending still not mature enough to be considered part of long term retirement planning strategy.
In addition, liquidating and moving IRA from LC custodian is pain and expensive. As the rules for secondary market trading in IRA are in flux, liquidating the IRA is not easily accomplished, can get expensive because of transfer fees charged multiple times by custodian and/or cash drag from waiting for loans to fully pay or default. LC/Prosper currently subsidizes the annual IRA maintenance fee so that lenders don't have to pay. But I don't know how long this subsidy will continue. The IRA custodian fees are too high otherwise.
Also, my taxes are already complicated enough with K-1s from MLPs and other partnerships, stock options and restricted stocks that adding more tax documents from p2p lending and FOLIOfn trading with regular taxable account is barely noticeable.