Both citations are appropriate for sole source follow-on procurements if the supporting documentation is adequate (but only one citation may be used in the J&A). Use (a)(2)(ii) for procurement of supplies and (a)(2)(iii) for procurement of services. Para (ii) talks to the continued development or production of major systems, if your level of effort contract supports development of a major system it could be appropriate. If it does not, you will likely want to consider using subpart (iii) which is strictly for services.
However your justification must support the services as highly specialized, that the duplication of cost is not expected to be recovered through competition, or unacceptable delays in fulfilling the requirements as provided in the (iii) subpart. The type of contract or funds is not really relevant to the justification for sole source - rather the justification needs to address the necessity of the overall action relative to the Government requirement.
I suggest you work with you program attorney to ensure you have adequate support for a sole source justification.