Never stayed in either ... and really I dont' think I want to .. the bars and nightclubs in them are bad enough ... who knows what the rooms are like.
If you don't want to stay in the Hilton then maybe it might be worth looking at B&Bs in the Ballsbridge/Donnybrook area - which is really very close to where the Hilton is - very short taxi or a walk ... they'd be in the same price range as the Harcourt Street Hotels, most probalby are nicer and wouldn't have a nightclub attached ! .. .No specific recommendations - but I'm sure there's info on Tripadvisor.
If you are going to an event int eh Hilton, chances are u will be out late and therefore not spending too long in your hotel so really you should just pick the most convenient to you.
You could try the Camden Court Hotel or O'Callaghan's Stephens Green Hotel, both within walking distance of the Hilton, cheaper than the Hilton though probably more expensieve than the Harcourt Hotel!
Sorry, don't know anything about the Rathmines hotels.
Stayed here in the Waterloo Guesthouse for € 45 for a Christmas party in mid December. The place was lovely and our room was palacial. I think we were very lucky with the room though.