Transfer from domestic Southwest to Delta Int'l flight
 

Hello all,
First post for me. I'm looking to book a flight St Louis to Dusseldorf in March 2019.

Best option I've found is STL - ATL - Dusseldorf. That brings up my dilemma: Delta flight (one ticket through) gets me to Atlanta at 5:56pm!!! for the 7:05pm departure nonstop to Dusseldorf.

I looked around and found a STL - ATL SW flight for $99 that gets me in at 5:15pm, and would also be available for the return. Plus the Delta flight to Dusseldorf is $350 cheaper from ATL.

Question is, risk it on Deltas short connection or risk on SW with a bit more time but probably a longer way and another check-in or security screening? I'd probably be ok with just carry-on in case that matters.

The delta connection is over an hour, I wouldn't call that "short" at all. If your SWA flight is delayed DL will do absolutely nothing to protect you. If your DL flight is delayed you're completely protected (they will most likely rebook you through CDG or AMS).

This is a no brainer.

You don't have to go through customs and immigration when leaving the US (it's basically like a domestic connection) and an hour for a connection through ATL is fine. If you miss your flight, Delta will protect if you are on a single ticket and there are multiple routing options through AMS and CDG that can still get you to DUS on the same day. If you are on separate tickets, you will basically be at Delta's mercy if you miss your flight and may have to pay for a new one.

As others have said, don't risk it with WN, and especially not for an "extra" 45 min (WN is certainly no better with on-time performance than DL). An hour in ATL is plenty, but if you're nervous about it, there are 5 or 6 earlier STL-ATL departures that can help bolster your layover time. They should typically price the same this far out. Picking a random March date, I see 6 options all priced the same.

You mentioned being OK with just a carry-on which is good since you would have to pickup your checked bag in ATL and recheck it if you went with Southwest. DL will check it all the way to your final destination.

Yes. What they said.
On another note, I had wanted to go to St. Louis, but on Delta it was simply very expensive.

DL might have some sympathy if you were flying another carrier from a station they don't service (not likely in ATL, but possible at some other gateways like LAX), but I wouldn't expect any relief when you chose another carrier on a head to head route to DL's Mega-hub.

While the DL connection is a lousy one for reasons I will explain below, the WN-DL ticket transfer ought to be a non-starter.

At a minimum, you are protected for a misconnect on DL. If you are delivered late by DL, it will be for DL to rebook you. If you no show at ATL on a separate ticket, your DL ticket may be cancelled and, depending on the fare rules, you may be required to purchase a new ticket at walk up prices. The $99 WN ticket becomes relatively meaningless and very few travel insurance policies cover trabsfers betwen tickets.

The DL connection while certainly easily doable in the inbound is on time becomes a mess if you misconnect. While DL will rebook you, you face either a 24-hour wait or being routed through other gateways in the US or Europe, e.g. JFK or CDG. During high season, last minute seats tend to be the worst on the aircraft and you face a 9-hour flight in a bad place. DL operates 6 nonstops a day on the STL-ATL hop and unless you have something really pressing in STL, I would take the earlier STL-ATL flight.

I'd also like to add that the domestic schedule for March 2019 is at best a suggestion right now. If the connection gets much tighter DL will switch you for free to the earlier connection.

I fly WN often, and have a future trip that is WN and then DL intl, but.... there is a day in between the flights.

I would NOT suggest what you are doing. Stick with Delta all the way. An hour in Atlanta is usually more than enough time.

EDIT TO ADD : Keep an eye of the flights after booking. If the times change, you can usually get booked on an earlier STL-ATL flight if that makes you feel better.

Honestly I can't imagine feeling that a 70 minute connection on an all-DL itinerary was "too tight" but somehow a 1:45 connection on separate tickets WN-DL was just hunky dory. That's crazy.

Agreed. A 70 minute connection on a once a day flight would feel tight. I'm with the OP there. But turning it in to a 100 minute connection that also requires switching airlines which means giving up misconnect protection... that's going completely the wrong direction for cost/benefit.

I've done a similar thing numerous times. Essentially a same day positioning to get to my TATL departure.
But.... I've given myself no less than 4 hours between flights. And I choose a carrier that has multiple flights to my departure airport. WN works well for me with lots of direct flights.
But in your situation, I'd stick with the single ticket and protection it affords.

I wouldn't want a 70 minute connection to a once a day flight either, but 100 isn't much better when you're changing carriers. When I do separate carriers (to LAX) I go the night before and get a hotel room. I'm usually saving more than $350 when I do it though.