Occasionally, I use both if I have seeds. Otherwise, just mulch. They have different purposes.
Cover crops are often nitrogen fixators, and are typically used when soil has nothing else growing in it, or with new plant transplants still too small to shade said cover crops. They also act as a "canary in a coalmine" so to speak, as the clovers/cover crop will show issues with watering and/or soil far before your main plants will. In some cases, they can even help with soil compaction. Of course, they die off when your main plants shade out the cover crops.
Clovers and the like are great for a variety of reasons, and although they're referred to as "living mulch" they don't quite do the same job as "mulch".
Mulch is a layer of material; woodchips, straw, hay/alfalfa/etc, and even compost. Mulch helps with water retention, makes even watering easier, and provides a nice home for a host of good critters.
Cover crops can help retain moisture, but not nearly as effective as woodchips or straw and the like.
Saw someone mention that cover crops can "take nutrients from your soil", however that is a fallacy. In fact, seeing as most cover crops are nitrogen fixators it is actually the opposite. The clovers will add nitrogen into your soil, and any minuscule amounts of nutrients they may have used will quickly be returned to the soil upon their decomposition. Sure, some cover craps can get fairly big, but look at their roots. Definitely not taking much from your soil.
As you can see, they both serve their own unique purposes and can in fact be used in tandem with one another.
One potential issue with mulch is that it tends to "tie up" some of the nitrogen from your soil. Mulch is carbon, and will decompose over time. Composting = C:N ratio, so, with such a large amount of carbon top dressed on your soil, some nitrogen will be leeched in order to decompose the mulch. Some Nitrogen will be tied up with the chips, not being released until the chips have fully decomposed back into the soil. Again, over time. Not the biggest deal, just means a little extra top dressing in some cases. However, still something worth knowing and considering.
In a no-till, or even a ROLS, this won't be much of an issue as your soil should have sufficient nitrogen levels in it even with mulch "tying up" some of it. Where this can be problematic is in brand new freshly made soil, which may not have enough Nitrogen to feed your plants once the N gets tied to your mulch.
People often believe only the plants need Nitrogen, however your soil itself uses it for energy, and anything decomposing will use it for energy (mulch included).
Why cover crops in tandem with mulch does so well, as you can gather from the info above, they balance each other out. Your nitrogen fixating cover crops will easily replace any Nitrogen tied up by your mulch. If cover crops aren't your thing (no biggie), you can still use mulch. Just might need to supplement with a little extra N in the form of extra top dresses, or a little liquid fish. For those of us that always have Neem Meal around to top dress with, you'll likely not notice this issue.
I'm never going without mulch, ever again. It is mandatory, especially outdoors. Not only do I now water much less, but watering evenly is so much easier. Peat is hydrophobic, right? No problem, soak your woodchips in water and come back in another hour or so. By then, the water will have "leeched" from the mulch and into your peat/substrate via wicking/capillary action. Now, you can water more effectively, as many dry spots will be eliminated.
Best part? Easy to find it free. I live in a small town (<2000 population), so not many places for me to just go and buy mulch. Woodchips from hardware stores are shit, especially the ones with the dyes in it. Not something I want leeching into my soil, stay away from those IMO.
Anyway, despite my super small town, I just found out this year we actually have a woodchip pile at our refuse station. Literally just drive up and shovel as many woodchips into my truck bed as I want, zero charge. If my small town has a free woodchip pile, then yours likely does too! Some even have compost piles, and in some cases leaf mould piles. Leaf mould is incredible, but I'd stay away from municipal compost piles personally. Unless you can verify with 100% certainty what is in the municipal compost, I'd stay away. Better safe than sorry.
Mulch combined with irrigation makes things incredible idiot proof.
All the best.
