I've stayed at 3 Nagas and Maison Souvannaphoum. Here are brief, subjective overviews of each property:

3 Nagas
The good: nice historical restoration, central location, competent service; reasonable fee for airport pick-up/drop-off
The not-so-good: thin walls (you will hear your neighbors), some street noise, no pool, average breakfast, more rice hawkers will descend upon you during the monk procession than at Maison Souvannaphoum
Bottom line: For one night and given your specs, it's perfectly fine.

Maison Souvannaphoum
The good: calm, quiet, great pool, graceful staff (who will go the extra mile for guests), good breakfast, generator in case of electrical outages
The not-so-good: overpriced airport pick-up/drop-off (better to hire a cab or jumbo), rack rates have increased, a less-than-optimal, though not inconvenient, location: it's on the outskirts of town center, but it's less than a 5-minute walk to the night market and about a 10-minute walk to 3 Nagas (at a leisurely LP pace); there's a large temple directly across the side street, so you can easily participate in the procession of monks (3 Nagas does a better job of marketing the monk procession; you can easily view or participate in the monk procession at Maison Souvannaphoum by walking across the street; if you inform the front desk the previous night, the kitchen will prepare a basket of sticky rice for you to distribute to the monks, for a small fee)
Bottom line: It's a good compromise between comfort and location and perhaps sufficiently cushy and quiet for your entire visit.

If you need more details, just ask.