Layover times
 

I am looking at flights from SFO to CDG. Some of them go via Helsinki with a 3+ hr. layover time, which stretches the travel time to 17 hours and have us arrive in Paris fairly late in the evening. I found two other options which shorten the travel time considerably and have us arrive in Paris late afternoon/early evening. One goes via Dublin with a 2 hour layover and the other via Chicago with a 1.5 hr. layover. In both cases it would be through booking with one airline. Is the layover time sufficient or should I choose to go over Helsinki?

If all flights are on one ticket and one airline, then if something interferes with the shorter connection time of your alternative flights, the airline will try to put you on the next available flight. Check these flights and see when the later flights will get you to paris as the worst case scenario, which could include a night in an airport. Then decide.

Of course, there are non-stop flights SFO to CDG, so I assume you are taking the longer itineraries in order to save money. Sometimes it is not worth it.

It's worth it when non-stop flights cost $6000 against $1050 for a one stop flight. Ironically, the return from Madrid can and will be non-stop in either case.

Would Dublin be a safer bet in case of a missed flight than Chicago?

I think any of those options should work. Personally, I don't think there would be much difference between going via Dublin and going via Chicago.

The non-stop non-refundable is often less than $1000 c/w Michael's 6K scare. Apples to apples, please.

I was thinking that there would be more frequent flights between Dublin and Paris than Chicago and Paris.

I don't know the layout of Chicago's O'Hare airport or terminals, but Dublin's AerLingus Terminal is fairly compact and easy to navigate - assuming that this connection would be AerLingus > AerLingus.
I'd feel very comfortable with a 2 hour layover in Dublin.
In addition, there are several flights to Paris, in case you miss the one you're booked on - as you wrote.

Well, that's the information I got using Matrix - ITA Software by Google Prices were for an open jaw ticket and I am not sure that in either case taxes and fees were included, but I would assume that the listings in that respect are apples to apples.

Sure. Just as there are more flights between SFO and ORD than between SFO and DUB. Somewhere in either itinerary, you have a single flight where, if something goes wrong, it hoses your plan. FWIW, I see only two Aer Lingus flights from DUB to Paris that work as connections from SFO. So you are really only gaining one "safety" flight and, if the problem turns out to be in DUB or CDG, or if the "safety" flight is oversold, you are still hosed.

Like I said, I don't see a huge difference between connecting at ORD vs DUB.

Two hours at the Dublin airport sounds like a comfortable transfer, with more margin of time than at the huge ORD. You want the itinerary on one ticket, preferably with airlines in the same alliance in case something goes wrong. Three things I weigh: time of departures, duration including transfers, and cost. I left the biggest to last. The prices on the Matrix are bottom-line, including taxes and fees (although not airline extras, which these days can add up.)

Assuming you’re on American or United the whole way through and your initial flight from San Fran is on time, 90 minutes should be enough time at O-Hare since you wouldn’t have to change terminals or go through customs.

For peace of mind, particularly when connecting in a large city, try to schedule a layover of at least 60 minutes.

We have changed at ORD for international flights with an hoir layover, no problems.

The non-stop non-refundable is often less than $1000 c/w Michael's 6K scare. Apples to apples, please.