This forum seemed to make the most sense to post this question, given I am talking about two *A carriers.

I recently purchased a flight on Air New Zealand in Business Premier (LAX-RAR). The flight departs late evening on a Saturday from LAX. I am considering buying a ticket on United to get from my home in Denver to LAX.

Option 1 (fly and connect same day)

I could buy a separate ticket RT (DEN-LAX) on United that gets me to LAX at a variety of times on Saturday (same day as departure for RAR). I may be able to interline my baggage and not leave airside at LAX. A premier phone agent told me UA will not interline the bags, but everyone else on Flyertalk says they will likely do this. I generally trust Flyertalk over United customer care. FYI, I am a humble Silver status with UA.

Option 1 Pro: Possibility to interline bags and not leave secured area.
Option 1 Con: Fingers crossed that nothing goes wrong with United on that day, because they only need to get me to LAX and not beyond (or care about my other flight).

Option 2 (position at LAX)

I could buy a separate ticket RT (DEN-LAX) on Southwest (considerably cheaper than United) for the Friday evening before my Saturday departure (LAX-RAR). I would need to buy a hotel for Friday night and kill a Saturday waiting on my international departure. However, the total cost here is about the same price as just buying the UA ticket in Option 1. The return flight is likely the bigger issues. Interlining luggage Air New Zealand to Southwest is not going to happen. I would need to go through Southwest check in and security. Oh, and did I mention the return is on the Saturday of Labor Day weekend at LAX? FYI, I have no status with Southwest.

Option 2 Pro: Piece of mind arriving LAX the evening before the international flight. I also get to start vacation a day sooner by hanging at the pool in LA before my flight.
Option 2 Con: Dealing with LAX and Southwest check in/security lines on Labor Day weekend Saturday.

So, what would you do? I appreciate any advice.

Even if you go for Option 2 for the outbound, nothing says you can't book the return on UA eliminating one of your perceived issues with Southwest. Most pricing of airfares in the US is based on one way fares anyways.

As LAX will be your port of entry to USA following the RAR-LAX flight you will have to go through USA immigration & customs, with all your carryon on & checked baggage.

What time of day do these flight arrive & depart?

+1
WN outbound, UA return. These tickets are priced on a one-way basis, so you will pay no penalty.

Based on the flight options for the return, we would have plenty of time to clear immigration and customs. Arriving 12:30 and the earliest nonstop departure to Denver is 16:10 on United or 17:55 on Southwest. My concern would be that apparently LAX was a mess last Labor Day weekend. So, the check in and security at terminal 1 would be the potential issue. As others have suggested here, Southwest on the outbound and UA on the return is workable at very similar prices.

This is workable. The Southwest RT and Southwest (outbound) / United (return) are comparable in price.

My only question is would you position the night before or fly from DEN to LAX the same day as the international flight?

Thanks.

I would position the day before. LAX has been a clusterflog lately with late flights, etc.

If I had the time spare, I would definitely fly in the night before. Takes all the risk and stress out of the equation.

Absolutely. Find a nice hotel in one of the beach cities and spend a relaxing day there.

Thanks all for the responses and information.

I was able to book UA round trip for just a little over the Southwest/UA combo price. I have also booked to arrive the day before. I am now searching for a nice hotel to relax at prior to the international flight.

Thanks again.