Six Grapes is not that good. At the lower end of the price spectrum, you are better off buying 10 or 20 year Tawny, or an LBV. Serving port correct is also important. LBV's should be run through cheesecloth. Port should be served just below room temperature.
Madeira is much better pick than port wine for cigars. Madeira has more acid in it and really compliments a cigar better. My first pick for reasonable Madeira would be Broadbent's 10 year or Leacock's Bual.
My picks for Port in what I assume is your price range:
Ramos Pinto 10 year tawny
Ramos Pinto LBV
Warre LBV
Warre Optimo Kirkland/Costco 10 year tawny
Delaforce Eminence 10 year tawny
Ferreira 10 year tawny