The engineer, especially where there's a great relationship, will tell it as it is, including their own frustrations with trying to get resolution on problems they constantly feed back, warts and all.
The sales rep will not understand, or even be particularly interested in the detail of the issue - they were likely selling houses a couple of years ago and will be selling cars in a couple of years time) - they'll dilute it, sugar coat it in BS and reply in marketing speak.
The really sad part of this is, their employer (typically the OEM) will think the engineer is a loose cannon with a big mouth, whilst the sales rep is a superstar.
Edit: I should just say this is (stereo)typical and of course there are exceptions, including some excellent ex-sales people who may be on this forum in a different capacity nowadays.