IMO Grosjean should be given the seat, sure he had hes chance and failed some say i think that chance wasn't exactly fair, thrown into a struggling midfield Renault mid season with Alonso as team mate with next to no testing in the car is a very harsh position(except for Kobayashi but hes an exception in this case).
Grosjean is literally destroying GP2 and has an incredible career Pre 2009 f1 appearance and post as well, i mean hes french and with Renault(yes i know the team isn't exactly works anymore but hes backed by Renault the manufacturer in there driver program) isn't that enough reason for them to back him.
Senna is just going to be a dissapointment.
IMO its either Grosjean(with the intention of using him next year regardless if he sucks to build hes experience, this also implying Kubica doesn't come back) or stay with Heidfeld, because There is no way Senna will catch up to Heidfeld levels before the end of the season and its just going to cost the team a chance to catch Mercedes in the constructors, i think Heidfeld has been hampered with an inconsistent car(just look at there hot and cold performances this year) and made worse by the fact he had little time to adjust to it at pre season.