If Gmail is the only Google service you use, then a WP will be fine. You say "stick with WP." Do you already have one? Since 2 of the 3 phones you mentioned are Android, I'm assuming you're not stuck on a platform. I don't have any experience with the Xperia SL, but my wife has a Moto X and I have a Lumia 920 (2nd device, not my daily driver). The Moto X is faster, smoother, and every bit as stable, if not more. It's also much more capable. The voice control is wonderful!
The 920 has wireless charging, which is nice. The Moto X doesn't have wireless charging, but it has excellent battery life. My wife gets up to 7 hours screen on time between charges.
The 920 is now a year old, and therefore cheap to buy off contract if you get a used device. Earlier this week I picked up a mint condition red AT&T version for $125. New ones are around $325 to $350. If you're thinking Moto X, you can get a 32GB for $399 if you can get a Today Show promo code.
Both devices & OS's have their advantages & disadvantages. Take your OS preference and budget into consideration, and go from there. You're well set no matter which of the 3 devices you choose!