I've stayed at both somewhat recently (last at the Fremont in April, the Queens in November). Both have decent rooms with smallish bathrooms. Both are central for downtown (IMO). 4Q rooms tend to be cheaper. They also have the better coffee shop. The Fremont has a buffet, some dinner specials, and Tony Roma's. And a Dunkin' Donuts. Both have competent personnel without making a fetish of it. The 4Q has had some dealers that are very quiet. Their casino is brighter. Both have good video poker, though not the best. 5x odds @ 4Q craps, compared to 2x @ the Fremont. But the latter seems to more consistently have a game going. $3 min @ at least one craps table and some BJ tables at the Fremont. I think all their BJ games pay 3:2, whereas most of the lower min tables @ the 4Q (under $10) are 6:5 or worse. North Tower @ 4Q can be noisy during the Experience, South Tower less so. I've never gotten any mailings or comps from the 4Q, except $5 earned from play over the years (they have a nice cash/comp earn rate; I'll let others tell you about it, since I'm unsure of the details). I got one free night offer from the Fremont (which is what I used when I went) and have received offers from other Boyd properties, which I usually play at more.
Sorry for the ramble, but I'd say for the same price, it's six of one and half a dozen of the other. But be sure to visit both, since they're right across the street from each other.