I see there are a slew of posts about taking the train, and I've looked at a lot of them, and also the websites like bahn and nshispeed, and raileurope, and others- but I'm still not sure:
We'll be arriving in Frankfurt during the last week in May, and after spending a few days there will head to Amsterdam to see daughter.

Is it better to book the tickets ahead?

How do we decide which site to use?

The fares seem to go from 39E to over 100E- why the big swing in prices? There's even a huge difference in how long the trip takes.

Also, first class is of course more than second- I have a very tall husband, so for a 4 hr trip I thought maybe first class for the leg room- correct assumption?

Thanks!

The fastest train connections are generally the most expensive. Fares for slow trains, and multiple connections are less. The €39 fares are book ahead, non-refundable fares - a lot like airplanes.

Unlike airplanes, train seats are not cramped. In most cases, 2nd class will have sufficient leg room. First class seats are wider, better uphostered..

The only time I have booked ahead was to take advantage of discount fares.

There are no better fares than those on the Bahn website.

The 39€ fares are book-ahead, non-refundable tickets valid for one particular train connection which are extremely limtied in number. There are only a few per train. They are more an advertising gag than a real offer and hard to come by. They must be booked well in advance. Tickets are available from 89 days prior to travel date, and these cheap discounts will be gone quickly.

Given the the Dutch are the tallest people in the world you husband will have enough legroom even in 2nd class. Unless you want slightly better upholstery I wouldn't waste your money on 1st class for such a trip.
The trip will take a different length of time depending on the routing. Not all departures take the same route.

ok, so it sounds like there's a few 39E seats, but most seats will be more- if those seats come up for our travel date, is there any reason not to grab them?

And to clarify, if I see the same fare on different sites, does it matter which one I book the tickets on? Sounds like Bahn is the preferable one?

Since you will be in Frankfurt before, you can plan on taking one particular train, so there is no reason against grabbing these tickets if you get them. I am pessimistic as this is only a month in advance. If discount fares show up in the timetable information this does not necessarily mean that they are really still available. Click "Check availability", and if they are, grab them. Good luck.

I will dispute there being little difference between the two classes other than better upholstery or wider seats

I have ridden German trains annually for 40 years and IMO there is a lot of difference between classes - 3 seats in the space of 4 in 2nd class - more chance of getting an aisle seat where tall folks can stick their long legs out and often empty seats in 1st class but 2nd, more and more full i think because of the online cheap tickets meant to guarantee a full train car so bahn.de is not ferrying empty seats around

But 1st class i always get an aisle and a window seat - so-called isolated seat just by boarding without any reservation (i always have a first class pass so the money issue is not there or i may go 2nd class because i'm on a tight budget) - in first class i always seem to be able to put my luggage on a seat next to me or across the aisle, etc.

so that's my take on the difference between classes.

Palenque, I think my husband would definitely agree with you, as far as having a place to stick your legs out!

And I am chuckling, not in a good way, quokka, as the cheap fares that were listed on some of the trains, you're right- when I click and go further, they are not available!

And then I went to nshispeed, and saw some good fares, but as I read elsewhere, the booking page switches to Dutch. aarghh!

Well if you go 2nd class then ask to reserve an aisle seat when buying your ticket - not sure if that is possible on the 39 euro fares - seat selection or not

reservations are not required on most German trains so you are free to move around once aboard too - reservations are optional with full fare tickets and a few euros extra - in 2nd class i would reserve to get an aisle seat for daddy long legs.

I can't say for sure about the Europe-Spezial (international) fares, but I did rather easily find Dauer-Spezial fare for travel within Germany at the lowest fares last November. I would not call them an "advertising gag", the are usually available for all but the most popular routes. But whatever, look and see if they are there, and if they are, use them. Simple!