I am planning to spend five days in Cornwall in August with my husband and two grown daughters. We are trying to decide between hotels in Penzance and Marazion. Does anyone have any suggestions about which would be nicer or more convenient or interesting? We may rent a car for a couple of days but don't want to have to drive everywhere.

Marazion is just a few minutes drive from Penzance on the other side of Mounts Bay.

There's hardly any public transportation from Marazion. If you want to use public transportation to the other towns, stay in Penzance.

Although it's not Penzance, we enjoyed Hotel Tresanton in St. Mawes. I believe the 60's model Jean Shrimpton owns a B&B in or near Penzance.

"Hardly any public transport from Marazion" ? Nonsense!
A glance at last summer's Cornwall Public Transport Guide (available free from tourist offices) shows buses to Penzance and Helston twice an hour, less frequent services to Truro or Falmouth, and an open-top bus hourly to Penzance, and every two hours to Land's End and St Ives.
Penzance has more frequent buses as well as trains. In August in Cornwall, the roads get congested and tourist towns like St Ives have park-and-ride services: you park your car in a field and take a bus into town.

Living in Cornwall, I would definitely stay in P'zance; you'll find much more to do in the day, and a very wide range of restaurants and pubs at night, plus you will easily be able to get to Marazion to visit St. Michael's Mount, which is really the only thing to do in Marazion.
In P'zance, don't miss the Penlee museum and art gallery with its beautiful newlyn school collection of paintings in the sub-tropical morab gardens [and there are some nice small hotels near-by too, and the walk to Newlyn itself, and possibly Mousehole [pronounced "mousel"]if you are feeling energetic. And that's just for starters. A car would let you explore the countryside which might be nice for part of the time, like St. Ives [though you can get the train from P'z, via st. Erth] Porthcurno, for the amazing outdoor minack theatre [try www.minack.co.uk for tickets], Sennen Cove, etc.etc.