Use HM-10 (Bluetooth LE) or ESP8266 for telemetry
BLE is much more power effiecient than WiFi. According to my experience with nRF chips vs ESP8266 difference is at least 5 times, closer to 10x when actively sending.
WiFi on the other hand provides higher bandwidth.
Connection issues you'll have at any rate, because BLE also has to find device and pair to it, albeit it's much faster than getting DHCP addresses and setting up a full-blown link. Setting up direct P2P connection with static addresses might speed up things for WiFi.
WiFi "field problems" are exactly the same as with BLE - both need somewhere to send data to. Most of the modern phones/laptops support both protocols, so you can set up an AP or BLE Central on phone and use that.

Basically it all boils down to amount of data you want to send versus amount of battery resource you're willing to pay for it.

P.S. you can send telemetry as ASCII as well, you just have to take into account that every digit takes a full byte. You can stuff your data into binaries and send them over Wifi as well in such a manner.