They are apples and oranges in a sense, but both Saigas and Yugos are very good, provided you get a Zastava factory-built Yugo rifle like an OPAP or NPAP instead of a US parts-kit build. Also avoid the "1st gen" PAP rifles that featured a single-stack bolt.
The Saiga will have a chrome-lined, cold-hammer-forged barrel, while the Yugos will have a non-chrome-lined barrel. But that's the way the Yugoslavians always made them, and I've never heard of anyone having an issue with them.
The finish of the Yugo OPAP and NPAP will certainly look better than the paint over parkerization finish of the Saiga, but the Saigas do resist corrosion very well. The machining of the Zastava rifles is pretty nice for an AK.
Saigas have a 1mm stamped receiver, as do Yugo NPAPs. Yugo OPAPs have a 1.5mm receiver and RPK-style reinforcements at the front trunnion. Personally, I feel that all the thicker receiver adds is weight. Saigas tend to be very accurate AKs despite their light weight.
Most AKM accessories and furniture will fit a Saiga. Not so for the Yugo rifles. That said, companies are constantly introducing new products for the Yugo rifles due to their increasing popularity.
Bottom line - you can't go wrong with a properly-converted Izhmash Saiga or Zastava-built NPAP/OPAP in my opinion.