We will be taking a train from Frankfurt (MAIN) Hbf to Amsterdam Centraal in July leaving at 9:30 on a Wednesday morning.

What are we better off doing:

a) Taking an Uber / Taxi from our hotel not far from the train station in Mainz to Frankfurt (MAIN) Hbf or

b) Going to the train station in Mainz and taking a direct 8:27 train to Frankfurt Hbf that arrives at 9:02.

Cost is not part of the consideration.

Our only consideration is the comfort of a car vs knowing what time we will get to the Frankfurt station.

If we choose the car route how much time should we allow to get to Frankfurt (MAIN) Hbf at that time on a week day?

If you think the comfort of a car outweighs changing trains as well as possible traffic delays then use the car. Do you know how long it would normally take the car to get to the Frankfurt station?

Personally, I'd rather be in the rail system from the beginning but that's just me.

Do you really think that the way from Frankfurt Hbf taxi stop to Frankfurt Hbf track 18 would be shorter than the way from track 4 to track 7 at Frankfurt Flughafen Fernbahnhof? Even the way from track 4 to track 5 at Cologne Hbf would have been shorter.

We're not debating which train to take. I have already purchased the tickets from Frankfurt Hbf to Amsterdam.

No change once we start so only get on and off the train with luggage once.

Get on at the start of the route so no problem with luggage storage.

What we are exploring is which method of transportation to the Frankfurt Hbf train station from Mainz is better for us (a couple in our mid-seventies though in very good condition).

I would take the train. You are far less likely to suffer a hold up, and the walk at Frankfurt will be less. It's probably quicker too. Of course we have no idea how far from your hotel is from the station in Mainz.
If you decide on a taxi make sure you allow leeway for traffic holdups and for getting to the platform from teh taxi stand.

" No change once we start so only get on and off the train with luggage once"
OK. So you give yourself the answer to your question.
If you want to board only 1 train, you cannot do anyhing else than a taxi ride from your hotel at Mainz to Frankfurt Hbf.

We are not at the Mainz train station but very close.

Google maps has it as an 8 or 9 minute walk and a 4 minute car ride.

How long would you allow for a car ride from Mainz train station to the Frankfurt Hbf train station at 8:00 AM on a weekday?

A tough choice. They are "nominally" about 40 minute trip each.

Ordinary RE and S trains doing this route are commuter trains near the end point heading to the work center. They would be packed standing room only trains with no place for luggage for 40 minutes. It was precisely the reason you were waffling between catching trains during their middle of the runs vs. flying. Also, in my experience, the trains going into Frankfurt HBF seem to get held up often just before reaching the station.

A car trip comes with trip time uncertainty as mentioned. It can be a smooth sailing - what's the deal day. You can also get held up in a major way due to many reasons.

Car with bags and leave early and then have a coffee at the Hauptbahnhof waiting for your train to come in.

Palenq,

I think that's what we'll do.

If we leave 7:45, that leaves us 1:30 to travel and 15 minutes at the train station.

Enough time?

15 minutes if you get there on time - why not leave a bit earlier if going by car - rush hour I would suspect. For one thing the Amsterdamned train may be sitting on platform more than 15 minutes early if your goal is to get there early to nab baggage space - I suspect you reserved seats - they do not automatically come with tickets I believe - could do a Mainz Hbf if not.

Palenq,

First class tickets come with reserved seats.

Our car has only one pair of single seats (nobody beside - just a window and an isle) that face each other with a table between. That's why I booked them as early as I did.

I don't want to get up early enough to get there at 6 or 7AM.

Is allowing 1 1/2 hours to get to the train station enough?

30 km or 18 miles by car should not nearly take that long even in worst scenario Mainz - Hauptbahnhof. You're set to roll.

Great. Thanks

" First class tickets come with reserved seats"

of course not! But if you have booked your seats and got the car number and the seat numbers, there will be no problem.
The IC to Amstardam originate at Ffurt Hbf. You may board it a certain time before departure.

Yes first-class tickets do not automatically come with seat reservations so check our ticket like neckervd says.

I booked the seats as part of the ticket purchasing process. There was no extra charge.

My ticket has the car and seat numbers printed on it. Shows on my app as well.
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Went through a bit of trouble. Last time I was away I had no trouble with my credit card. But when I make "out of the ordinary" purchases they stop the purchase and text me to reply if it's ok. And rather than continue with the purchase they tell me to redo it. Great!!! What we put up with for security.
Well, I replied but the purchase kept looping around and wouldn't go through. I finally gave up and cancelled it. Then I started over with the intention of using PayPal. But now the seats were locked up and unavailable.
I yelled at my credit card company but they didn't really care.
I emailed the railway and they couldn't do anything.
About an hour later I tried again and by then the seats were released. So I made the purchase using PayPal. All's well that. . .