You do not need to care about UFX+ thunderbolt speed, thunderbolt will automatically adapt to the proper speed for connected devices.

Its IMHO irrelevant for audio use cases to compare PCIe sockets, whether they are x1 or x4. Thunderbolt needs more lanes because you run several devices which needs more i/o bandwidth and thus more PCIe lanes. The HDSPe MADI FX card simply focuses on audio transfer for the 3 MADI busses.

The advantage of the HDSPe MADI FX card is
- that its driver uses only system resources for the amount of channels that are in use (*)
- the distance between PC and connected audio gear can be much bigger, MADI: 2km, TB: 3m
- you can select between more mainboards if you do not require thunderbolt
- one TM FX instance for all devices on all 3 MADI busses and very flexible routing
(but you might need an additional instance for recording devices like the UFX+ to operate it up to the point where you can use stand-alone mode)

In contrast to that the MADIface USB3 driver of the UFX+ allocates system resources for every audio channel of the UFX+ no matter whether its in use or not. USB has more protocol overhead compared to PCIe and requires tad a bit more CPU for processing data.

Here a quick calculation / comparison of RTTs of different RME products "Summe Latency".
Then two additional scenarios:
- UFX connected to RayDAT (RTT from Mic to i.e. Phones or speakers)
- Octamic XTC connected to HDSPe MADI FX (RTT from Mic to phones output of XTC)
In both cases you need to add time for AD and DA conversion to get the complete RTT ("from mouth to ear")
The very fine RME manual (thanks Matthias) delivers to you the data (screenshot copy pasted to the Excel sheet)

You will notice, each of the solutions is not far from each other.

The rule of thumb for taking USB or Firewire was, take what works best in your system (depending on mainboard design and  chipset).

The rule of thumb for taking USB3 or Thunderbolt or PCIe is, take what works best in your setup.

If you work with a mix of UFX+ and ADI-2 Pro, then taking the MADIface driver might be a good choice, because it suports both devices UFX+ and ADI-2 Pro, then your DAW can access them both directly (they need to be clock synced and to use same ASIO buffer size). If your mainboard has USB issues then you can use the Sonnect USB3 card offering 4x Fresco 1100 USB3 controller chips. The card uses MSI (message shared interrupts) which work more efficient, esp under heavier system / interrupt load. This fully isolates UFX+ / ADI-2 Pro, etc from the rest of the USB infrastructure of your mainboard. Only requires a free PCIe x4 socket.

I have no experience with the UFX+ thunderbolt driver. I only assume it also has to allocate system resources for every audio channel. But as TB is directly attached to PCIe and simply maps the channels to each other, I think this has not so much impact. This driver also supports pitch (which is not possible with the USB3 transfer mode). You are limited by TB cabling, which may only be 3m long. Expect the higher potential of interruptions if you chain devices. This is what I hate with such chains. I would use TB only to connect audio or a PCIe expansion case.

The HDSPe MADI FX is IMHO the best and most efficient solution in terms of
- connects directly to PCIe
- offers 3 MADI busses and routing between MADI busses
- offers two MADI redundancy modes
- offers driver optimizations to only allocate system resources for audio channels that are in use
- offers FX chip on board
- has very nice analog I/O and AES on board
- can connect i.e. an ADI-2 Pro/* to the AES port on bord for even Reference Grade AD/DA conversion
- IMHO the best and most attractive PCIe card including FX

The UFX+ is IMHO the best and most efficient solution in terms of
- you reach the lowest RTT for all analog I/O which is directly attached to the device
- offers DUREC which is nice to work like with a tape deck
- is fully stand-alone operateable via Display and 6 user profiles
- connects to either USB3 or TB
- offers FX chip on board
- offers 1 MADI bus for expansion

I am using UFX+ + 2 XTC but also bought the HDSPe MADI FX because I am interested to get lowest system load.
Especially as I discovered how much performance the Spectre and Meltdown CPU microcode upgrades suck.
For this reason I backed those upgrades out by downgrading the BIOS.
But I still like the HDSPe MADI FX as I like this design idea that the card only allocates resources for audio being used.

So I can connect now the 2 XTC to either one (with automatic latency compensation) or two (without latency compensation) MADI busses. The UFX+ use cases are that I have by this still more analog I/O ports and quickly accessable ports in the front. And I can use Durec as Tape recorder or for recording backup purposes. All of this is possible at the same time.
For the HDSPe MADI FX Global record is the Durec directly on PC.
The advantage of Durec Backup is to have it isolated from PC on the unit itself.

Lots of possibilities. There is no "wrong" solution for you, as everything simply works great.

And if you take a look, even when using UFX+ via USB3 you can still easily use it for very big number of tracks and VSTs:
https://www.tonstudio-forum.de/blog/ind … cks-de-en/

The isolation of UFX+ and i.e. ADI-2 Pro by using the Sonnet Alegro Pro works great and I personally think MSI based interrupts are also a great improvement if you have a high system / interrupt load.
https://www.heise.de/preisvergleich/son … 79334.html

The reason to consider were that instabilities started with the mix of USB devices, it seems it overloaded USB or impacted each other on the USB bus coming from chipset: 2x UFX+, ADI-2 Pro, USB3 hub, Bluetooth USB stick, etc ...