Bus from Rome to Florence

Those prices seem high. On the Trenitalia site I checked a few days 2-3 weeks from now and found tickets for as low as €12.90 pp. Trainline won't always have all options available. Book either on the Trenitalia site, or check the Italotreno site for their trains.

Of course if your budget is severely limited, then go with the bus. You can check their reviews on Trustpilot. They don't look great to me.

There are train tickets as low as E10 pp depending on your dates.
The ITAbus goes from Tiburtina Station in Rome, quite a way from the Centre of Rome, so you’d have to get out to there ; then it stops at a place quite a way out of Central Florence .
By the time you’ve found local buses (and never assume….), or found (hopefully) and paid for taxis to get you to and from both places, I think the train is a much better choice.

As Jean said, only book trains through Italo or Trenitalia, never on Trainline.
Both have very good websites and apps.
The train stops at SM Novella Station in Florence, then you can just walk to your accommodation.

Let us know what you decide, and how it goes on your trip!

Edit!
I have just seen that you can get a tram in to SM Novella from Villa Costanza, so that would be very easy if you decide on the ITAbus.

Trainline is a reseller that may not be offering all your choices

Book your tickets in the official train sites
Both gave easy to use apps

https://www.trenitalia.com/content/tcom/en.html
https://www.italotreno.it/en

I checked the other recommended sites. Italo had the better prices for what I was looking for. But what is wrong with Trainline? I've been using Trainline because it was recommended on this forum! I am booking trains both in a single country and across borders.

The Forum members who have and do travel to Europe and use trains would not recommend Trainline or any other third party ticket reseller.
The reason is, as previously stated, is that you will get every single journey option there is for Italy with Italo and Trenitalia, and you do not with Trainline….some will be missing.
I don’t know about other countries.

It will also cost more.
You can find some great deals with the actual train operators on a lot of routes!

The Forum members who have and do travel to Europe and use trains
would not recommend Trainline or any other third party ticket
reseller.

I'm a Forum member and have and do travel to Europe and use trains and I often use Trainline. Find it incredibly convenient.

I wouldn't bother with the bus from Rome to Florence, as it would be a waste of valuable holiday time. Travel time on the high speed trains from Rome to Florence is only about 90 minutes. I normally use a taxi from my hotel to Roma Termini station.

I've used Trainline in the past and found it very convenient and user friendly, and wouldn't have any hesitation using it again. As others mentioned, they're a reseller, so may charge a small service fee, but I've never found that to be a problem. If your dates are a long way in the future, Trainline may not have all the information posted yet. I'd suggest also checking the Trenitalia and Italo Treno websites.

The high speed trains such as the Freccia will be slightly more expensive, as they require compulsory seat reservations.

I've been using Trainline because it was recommended on this forum! I am booking trains both in a single country and across borders.

I have used Trainline when I had trouble with a credit card on a train company’s site. I seem to recall it being very helpful on a couple of French itineraries and it can be worth checking for comparison. However, the Italian train companies’ sites work well and offer the best value in that country.

I can book bus tickets for 3 for $3.37

I find that one difficult to understand. $3.37 is close to 3.11€ or 3.12€ for 3, or 4 cents over one Euro per ticket. And that's with the Trainline fees built in. Really?

I checked a few days 2-3 weeks from now and found tickets for as low
as €12.90 pp.

I'm seeing the same as Cjean on the Trenitalia website so I don't know why Trainline is showing $80-$110 for 3. Then again we don't know what date/time you're looking at? Anyway, If it were me and my hotel was walking distance to SM Novella and I wanted to get into central Rome, I'd sure go with the train. Getting there in roughly an hour and 1/2 versus 3+ hours (not including getting to and from possibly inconvenient bus stations) provides more valuable sightseeing time. No chance of an accident or other road issue causing delay either. Well worth €38.70 or more.

Itabus makes two stops in Florence. One is at the Villa Costanza Parking along the A1 freeway. The other is on Piazzale Montelungo, which is between the SMN station (track 16) and the Fortress called Fortezza da Basso.
From Villa Costanza you can take the light rail tramway to SMN. From the Fortezza da Basso there are escalators to the track 16 then a walk to the main building of the SMN. There are also buses that go to Piazza San Marco but I think walking might be faster if you are headed to the station.