Passport control at FRA and BRU

there is no passport control for travel within the Schengen zone

Perhaps I should clarify. I am leaving from USA and stopping in either Frankfurt or Brussels. Final destination is Poland.

Ben, assuming this is one through booking and you do not have to check in again for the second flight

• You enter the Schengen Zone at Frankfurt/Brussels, whichever one you actually go through. You will go through immigration there and your passport will be checked and stamped.
• There is no passport checks for your second flight as it is within the Schengen Area.
• Your luggage will be labelled through to your final destination (Poland). You will pick it up there and carry it through customs (random checks only). The label on your luggage will identify it as from outside the EU.

So it doesn't make any difference whether you change flights in Frankfurt or Brussels.
The normal rule is that the airline would not have shown this as a valid connection if it were not possible. 90 minutes should be plenty. 60 minutes would be tight but do-able with no delays on the incoming flight.

All things being equal (like airfare), I would choose FRA over BRU any day.

I would with have a one hour and a half or one hour layover depending
on which flight I choose. Would that be enough time?

No. If your inbound flight from the US is late, your connection will leave without you. The next available flight for you to the destination city in Poland may be the next day or the day after that depending upon availability of seats. Do you want to risk that?

Poland is not an airport, its country with a number of airports. On Continentals tack, how many flights are scheduled to your destination airport, preferably on the same partner airlines.

For instance, from Frankfurt, to Warsaw, Lufthansa/LOT has 6 flights on weekdays, 8:10, 10:30, 12:10, 4:35, 7:50 and 8:40. So if you were delayed and missed your intended connection, you would have later flights that day they could get you on.

At Brussels, Brussels Airlines has 3 flights a day, LOT also has 3 flights a day, but I don't know if they would put you on a competing airline's flights.

@ChrisF: Two weeks ago, I flew from IAD (Washington Dulles) to FRA, on the way to Edinburgh. I actually did not go through passport control (immigration) upon arrival in FRA; just got off the plane and followed the 'connecting flights' sign. I did have to go through security again in FRA, on my way to the departure gate for the Edinburgh flight, but never went through passport control. Of course, when I arrived in Edinburgh, I went through immigration there. I think this was the first trip that I didn't go right to the passport control upon arrival (with connections).

Robert

that's because your destination was not the Schengen zone but the UK. You didn't technically enter the Schengen, simply transferred flights. Passport control was at your point of entry to the UK. The Schengen, the EU and Europe are not all the same.

I prefer to connect through Frankfurt whenever possible, find that it's a very efficient airport and have made my connections easily. I wouldn't do less than 1 1/2 hours, though.