You should go with the TM reading is what I am told. Always better to be a little over per the watch then short on the run. It doesn't seem like much, but a half mile here and there over the course of 5 runs a week leaves you 2.5 mile short of the plan that week and on average 10 miles a month. So it adds up. Also the mechanism in the watch isn't perfect. If it works in similar fashion as my garmin, it needs several outdoor runs to get it calculated. if you just opened the box and put it on and went run on the TM I think the difference would be a lot different as well. Also, if its like my Garmin, you really uses your last outdoor run as the main determining factor for pace / miles. It calculates your arm swings into a pace plus time with that many swings of the arm. So lets say you go run 5 miles at a goal pace of 7:30/mile. The next day its snowing 9", so the TM it is. Well the plan is a easy 3 miles, so you set the TM to a 8:45 pace. What happens (especially for the first mile) is your watch will hit mile one before the TM and will give you a mile pace of 8:00 ish lets say. To continue on, the longer the run, the more accurate it gets. Meaning, mile one will be the biggest difference you will have in real pace vs watch. Lets use our example above where you have a 45 second difference. Well mile 2 will get closer to the TM pace by a few second, and so on and so forth. The same is true for the reverse, run slow outside and do a faster pace on the TM and it will go the opposite way. Just not as much as a difference as the other way. The best way to track a TM run with a watch is to get the foot pod and calibrate it. One last point, if your pace varies on the TM, the watch doesn't always do a great job on just noticing .1 to .3 mph increase. It will show a difference just not as accurately if you are doing the run outdoors.
The statements above are based on my observations with my watch which is a Garmin 235. Not trying to say don't buy this watch, I think all of them using similar technology more than likely has the same issue. Just something to be mindful about.
Sorry about the wall of text.
A few extra steps, sure. But if it can help even just one person with a problem related to running instead of just giving up, I think it worth it. I think it would be something I update on a weekly basis perhaps...
Nailed it!