So - which system is best for use with FD lenses?

Neither EF nor MFT are ideal for adapting FD lenses.

• To adapt FD to EF requires corrective optics with a 1.4x multiplier.
• MFT has a 2x crop factor. With a 0.72x focal reducer, this can be converted to 1.44x.

If you intend to use adapted lenses extensively, consider an APS-C or full-frame mirrorless camera. Mirrorless cameras have shorter flange focal distances, which makes it easier to adapt foreign lenses.

• Full-frame is expensive, but the sensor matches the film format for which the lens was designed. Options: Sony, Nikon, Canon, Leica?

• With APS-C, focal reducers provide near-full-frame coverage (1.1x) with some trade offs (edge/corner sharpness, risk of "blue dot" flaring, other aberrations). Pairing a focal reducer with a plain adapter allows lenses to serve double duty (1.5x and 1.1x). Options: Sony, FujiFilm, Canon, Nikon?

Or are there too many downsides to using the old lenses completely?

Pros of using adapted lenses:

• Inexpensive – There is no need to limit yourself to only FD lenses.
• Image quality – Lenses have different "characters".
• Fashion – Old lenses look cool.

• Manual focus and aperture – Slow down and spend more time composing. Feel like a "real" photographer.

• Nostalgia – Use the glass you cut your teeth on.

• History – Relive the (photographic) dark ages with slow, uncoated optics.

• Two for one – Lenses can serve double duty when used with focal reducers and plain adapters. 50mm doubles as 36mm. 135mm doubles as 97mm.

Cons:

• Lens buying addiction and GAS

• Manual focus – Too slow for sports and birds?

• Manual aperture – Just pretend you're using aperture priority mode.

• Extra rings to attach to camera and lens. With enough adapters, you never have to remove them from the lenses.

• Lenses don't behave exactly as they did on film because of crop factors and sensor technologies. Like any other change, it just takes time to adjust.

• Some lenses may not be able to resolve to the level of the sensor. Canon FD lenses, especially the newer bayonet style, should work well enough for 24mp APS-C or 54mp full-frame sensors.

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I’m looking to buy a DSLR and I’m stuck between buying a Canon with an EF mount or a camera with Micro 4/3s.

There is no DSLR with MFT mount because there is no room for a mirror box.

I have a good set of old Canon FD lenses (From a 28mm to a telephoto) that I’d like to use, and ideally not buy many new lenses.

The best experience with any camera system is with native lenses. Consider obtaining at least a kit lens, which you can complement with adapted primes.

... if there’s a full frame sensor then the lenses will be closer to their original focal length?

Not necessarily. Suppose you were to adapt an FD lens for use with an EF mount. The flange focal distance of EF is greater than that of FD. Corrective optics that multiply the focal length by 1.4x would be needed. So a 50mm lens would behave like a 70mm lens. However, the same lens used with a focal reducer (0.72x) on an APS-C mirrorless camera (1.5x) will behave like a 54mm lens (in the original 135 format).