What are Mode C requirements with ADS-B 978?
For Class A,B and C, you'll need a Mode C transponder as per the present regulations (unless cleared by ATC). From 14 CFR §91.215 ATC transponder and altitude reporting equipment and use:

§91.215   ATC transponder and altitude reporting equipment and use.
(b) All airspace. Unless otherwise authorized or directed by ATC, no person may operate an aircraft in the airspace described in paragraphs (b)(1) through (b)(5) of this section, unless that aircraft is equipped with an operable coded radar beacon transponder ... and that aircraft is equipped with automatic pressure altitude reporting equipment having a Mode C capability that automatically replies to Mode C interrogations by transmitting pressure altitude information in 100-foot increments. This requirement applies—
(1) All aircraft. In Class A, Class B, and Class C airspace areas;

FAA also addresses this in its FAQs regarding ADS-B:

Will Mode C transponders be required indefinitely?
The FAA is considering additional changes in the national airspace system, such as for TCAS. These changes, may at some future date, reduce or eliminate the need for transponder equipage.

So, at present, you'll need Mode C anyway.
Also, you'll need 1090ES in Class A. From FAA FAQs:

Aircraft that fly internationally and aircraft that require TCAS (Traffic Collision Avoidance System) are already operating with Mode S transponders and many have older versions of ADS-B Out on 1090ES MHz. Aircraft flying in Class A airspace must operate on the 1090ES MHz frequency. General aviation users who choose the UAT (978 MHz) link may take advantage of the ADS-B traffic, weather, and aeronautical services that are transmitted on the UAT frequency at no charge but must retain their current ATCRBS transponder. Mode S, 1090-ES users can operate in all airspace, but cannot receive FIS-B services.

I agree with you that there are a number of services (through FIS-B) for 978 UAT. In that case, (if you are not going above 18,000 ft) I suggest you check the availability of UAT receivers before taking a decision.