Steam Hardware

Clonezilla should suffice for cloning the SSD.
A docking station (like this) with included m.2 slot for the Steam Deck should do the trick to have both SSDs attached to the Steam Deck - if you plan on having a docking station for your Steam Deck anyway, that is.
Without such a docking station you need an adapter (or even two) for the SSD(s).
Easiest way would be to swap the SSDs and install SteamOS anew.

I upgraded mine with a Corsair MP600 Mini 2 TB back when that didn't cost an arm and a leg.
Per GB SD cards aren't a lot cheaper, but typically are slower and have worse durability than SSDs.
I do have an SD card in my Steam Deck, but only use it for my gog.com library. It makes moving that library between my Steam Deck and my laptop easy. On that SD card are only smallish games.
I wouldn't want to run Cyberpunk 2077 from an SD card.

I run the 1tb SSD, a 1tb microsd and a 2tb USB-C SSD.

The internal SSD is all the normal Steam Deck stuff, the microsd is all emulation, and the external 2tb is a Windows 11 boot drive.

You want to upgrade the SSD.

I upgraded to a 2tb Corsair SSD (remember when you could just buy stuff? For reasonable money?). When I copied stuff over, I started out with the Corsair in an external enclosure. The card crashed during the copy, and I think the reason why is that these cards are dramless. They borrow ram from the host, and when you're in an enclosure the "host" has no ram to give. What I had to do was put the new card in the deck, install a steamos recovery on it, put the old card in a reader, and copy in from there.

Uhh I'll leave the details of copying to everyone else, but just know that writing to an external nvme enclosure can give you a bad time.

I would highly recommend upgrading the SSD. There is a major difference in download/install speed, update speed, and a notable improvement in loading times. Some newer games will struggle to run on a microSD at all, because they load assets in real time and the read speed is too slow.

I typically install small and older games to my microSD, but for anything modern or larger than a few GB I stick to my SSD.

For cloning mine when I upgraded, I actually just removed the old SSD from my deck, plugged it and the new SSD into my computer (didn't bother with screwing them down), and cloned them there. My main PC is linux, so it could clone the ext4/btrfs formatted drive without any additional software. If you do this with a windows PC you'll probably need special software. Another issue you can run into (especially with windows based cloning tools) is that many of them will clone the partitions exactly, meaning your new drive (despite being larger) only has partitions sized the same size as the original drive. On linux you can clone the partitions and then expand the partition to the full drive size, but I don't know if this can be done with the compatible windows clone tools.

Your best bet (if your main PC is windows) is probably to make a linux bootable drive (can be any linux distro with live boot and gparted/kparted, but Rescuezilla and Clonezilla are made specifically for this kind of thing) and boot the PC from that. That will give you full access to the linux tools for easily cloning and resizing the drive.

Alternatively, you may also just want to install the new SSD in the Deck, and reinstall steamOS using a steamOS recovery drive. It won't transfer over your files, but all your steam games/cloud saves should be easy to redownload. This is probably the easiest option.

I don't notice any difference from ssd or from sd card. I have SanDisk Ultra, 512 GB, 150 Mb/s

I got a deck with the small ssd and swapped it with a larger. I did not clone the drive, I installed the os via uSD. Other than that I have never used a uSD card in my deck.

I had the 64GB model, and I upgraded it to a 1TB SSD.

It is miles better; mostly because some things don't get installed in the microSD card (shader cache, proton versions...), so I barely had any space left in the internal storage, even though I had 512GB on the SD card.

I didn't clone the previous install, I just did a new install from scratch, but it was painless and very fast with a USB-C pendrive by following the official instructions. I had to redownload some games but it wasn't an issue with gigabit internet.

The performance is similar for most games if you have a quality SD card, but if a game does heavy loading you may notice a difference.

I actually did both but if you had to choose one it is without a doubt the SSD. And it actually is quite easy to do, just take your time.