Exercising help pls

I would input the calories in from your exercise bike, however most people agree that these readings from exercise aids are often over what has actually been burned so keep that in mind when/or if you eat your exercise calories back.
For example I only eat half of my exercise cals back or none at all if I'm not hungry to be on the safe side,

Neither of them are going to be massively accurate. The bike may be better if you have to put in your age & weight, otherwise it just assumes an average.
Whichever one you use, always use the same for consistency. Or split the difference between the 2?
I find MFP massively over-exaggerates my calorie burn for some exercises & is under for others. I use a heart rate monitor & log whatever that says.

I'd say the bike is more accurate since it probably calculates your wattage, cadence, resistance, etc. Going 15mph uphill burns more calories than going 18mph flat. If the bike asks for your age and weight, it will be even more accurate. Adding a heart monitor would give you even more accuracy. Does the bike have a heart rate monitor? Mine didn't come with one but I could add one.

If you do get a heart rate monitor (HRM), I would strongly suggest getting one with a chest strap. The watch only models aren't as accurate. As far as using the calories from your bike, it can really depend. If it asks for your age and weight, it can be fairly accurate but could still underestimate or overestimate. Most overestimate but the ones in my gym actually understimate compared to what my HRM tells me so you never know.

You didn't really say what you were getting from readings on MFP or the bike but let's say MFP gives you a huge number and the bike gives you a really low number. You could always take an average of the two. And try not to use the earned calories as an excuse to overeat. You don't want to try to net near your goal but there's nothing wrong with leaving a couple hundred calories uneaten, especially when you have more to lose.