SmartWool TML Light SportKnit 1/4-Zip Top - Long-Sleeve - Men's - 2010 BCS

I'm a merino nut, but that being said it depends on the activity. Merino is excellent when you are moving at least a bit, and especially in these lighter weight products. You need loft to keep warm if you're sitting around, and fleece will do that (or full on insulation synthetic/down). If you are going to wear this or the fleece layer when you are moving at all I'd go with merino though I'd actually aim at a heavier layer like midweight or heavyweight if you are looking for equivalent warmth. The bonus to merino is the temp regulation which means--barring extremes--you can't really wear too much of it. I've had on a heavyweight over a midweight and gone from 40's at the base of a small mountain up to 80's when we hit peak about 1500 feet of climb (Arizona winter) and was a bit sweaty but the merino works that like a champ. There's also durability in washing as fleece will pill up and thin out where merino keeps going.

I've rambled on in generic terms, hopefully someone has tried the exact two products you're trying to compare.

Warmth-very similar. I have this and wear it over a microweight layer for most ski days (or around town).
Breathability-WAY better. The other side of that is that it will let a bit more air in if you don't have a shell over it. Hope this helps.