Train Venice to Florence

They are two different models of trains, the newer "Silver Arrow" and the slightly older "Red Arrow". Probably not enough difference to go out of your way to pick one or the other, schedule would make more difference.

I would not be concerned about getting seats, but there may be a cost savings by buying now. Otherwise, buying a day or so ahead would be no problem.

what is the difference between the Frecciargento train and the Frecciarossa train?

Not much, they are different models of train and different branding. Both operated by the Italian National Railway company: Trenitalia.
For everything you ever wanted to know about travelling by train in Italy, read this web page: https://www.seat61.com/Italy-trains.htm

do I make the reservation before we leave for Italy to make sure we have seats, or will we be okay if we wait until we are in Venice?

Wrong reason. There are always seats available, right up to departure time.
But, if you buy early you can get the cheaper fares, If you buy on the day only full price tickets are available.
Venezia S. Lucia to Firenze S. M. Novella, cheapest fare €24.90, fare you pay if you buy on the day €50.

You don't make a reservation, you buy tickets. A ticket is for a seat on a specific train, it will say, date, time, train number, coach number and seat number(s) on it. Once you buy your tickets you can't change your mind and travel on a different train.

Buy your tickets from the Trenitalia website: http://www.trenitalia.com/tcom-en
They will sell you "print-at-home" tickets.

You can also take an Italo train, starting at €23.90 that day. Departure at 9 am or 1 pm, arriving two hours later.

Frecciarossa trains (red arrow) are newer than Frecciargento (Silver arrow) and run at an higher top speed: 300 kms per hour vs 250.
Not that there are many tracks between Venice and Florence where trains can run at 300, but using high speed trains on medium speed tracks is cheaper than building new lines.

Trenitalia is purchasing the new Frecciarossa1000 trains that run at 350 to be used on the Turin-Milan-Rome route, so they have some Frecciarossas to be "wasted" on the Venice line.

Thank you all for the replies. Can't wait for trip #4 to begin.

I'm a big fan of Trainline.eu. We used it with great success last month in Italy. Easy to use app with no fees. Everyone else was at the ticket machine or ticket office. I got mine standing on the platform. You can buy them in advance or last minute. No validation is required.

The Frecciarosso has 3 classes of seat layouts in 6 different categories, Standard, Premium (seats recline a bit and a few other adders), Business (larger seats 1+2), Business Saletino (compartments), Business Silenzio (no cell phone use), and Executive (super spacious). Frecciargento is just good old 1st and 2nd.