Sorry, I meant 2.5". Looks like I can't edit my post.
No, what I wrote is correct. Any good quality SD-card bought the last 10 years will be class 4 or higher. I have yet to find a card that does not work in the IDE<->SD-adapter I linked to. I have been using that exact adapter with ST's, STE's, Falcons, Firebee and Amigas since 2011 without any issues. Have used cards ranging from an ancient 32Mb card (still in use in one of my STE's) to 32Gb SDHC cards (Firebee), all of them just works. Have bought all of these cards locally in my little town, have not found anything less than class 10 cards in the shop for many years now. Speed is not an issue in a Falcon, any class 4 or 10 card is faster than what you can squeeze through the Falcon's IDE. A Falcon with heavily accelerated bus may be able to exceed the speed of a class 4 card though. My Firebee was capable of around 8Mb/s on the IDE port, I needed class 10 cards for that. CF-cards were not any faster.
The only downside with the adapter I linked to is that it use Cable select and not Master/Slave, so it needs hacking if you want to have two devices on the Falcon's IDE bus. However, in my Milan I have another IDE<->SD adapter (40 pin) which does have Master/Slave-jumpers and work perfectly fine with a second device.
With SD you don't have to search for the correct card, they all just works. I have not seen CF-cards in local shops for years, in the past you could buy them in photo/camerastores but nowadays you have to order them. And they cost 4 times as much as SD-cards of similar quality and capacity (Sandisk Ultra 32Gb SDHC - NOK79. Sandisk Extreme 32Gb CF - NOK348). There is IMO only one good reason for choosing CF over SD on a Falcon - if you already own one it would be cheaper than SD due to the lower cost of the IDE adapter.