Having considered between unsing my android phone as a bike computer and buying dedicated computer, I noticed some points that could serve you.
Using cell phone:
- You need a bike mount to be able to look at the data while you ride,
moreso for navigation, and they are somehow bulky, just like a cell phone compared to the dedicated bike gps you mention.
- Cell phone battery will drain quicker, specially with sensors attached.
- Usually, phone GPS is usually not as good, and for instance, I don't think I can connect to GLONASS satellites from my phone (Russian satellites for extra gps accuracy).
- Bluetooth only allows for one sensor connected, so if you want to link a heart rate band, you could not link a cadence sensor.
- If you want to connect sensors via Ant+ your phone need to be able to receive the signal, which is not the common thing, or buy an ant+ receiver
- Usually phone apps give their best stuff on payed premium plans.
- Phone apps usually take a good while deciding you stopped, which means a big time increase when measuring city commutes with all red lights etc. This can be avoided with speed sensor
Using dedicated bike computer
- There are less expensive units that can connect to a phone to use it as their gps sensor, while the unit serves as smaller cockpit display with ability to plug in more sensors (basically HR, cadence and (more accurate) speedometer), but they won't show navigation maps.
- Deciding between bike computers is already very complicated. The main difference is if you need navigating maps your options are less, and the better you need your navigation (i e you plan on using the gps unit on touring bike expeditions) the more clear it is that you need a big screen unit with plenty of battery time and good maps.
- If you really like strava you may consider some devices that show strava segments live, but keep in mind this needs strava premium membership.
- I found out something about gps recording: some units will record more data points of your traject than other. This could affect shorter strava segment times :P, but looks like it's not a big deal otherwise.
- Some units have barometric meters on them, offering more accurate elevation gain
- Some units can display a great amount of data on the same screen
- Some units have several training features that could be useful
I don't like and it seems it's not welcomed to give product recommendation, but since you mentioned it, I bought the gps unit you looked at. Still haven't used its follow track feature, but I bought it because of that. If you are not interested in maps, probably the competitor polar m450 is a better choice. For some reason I don't like the scheme that goes "no gps bike computer with connection to phone for gps" but it doesn't look like a really bad idea and it is way cheaper. Since I wanted a christmas gift i didnt check the cell phone bike mount, but I would have given it a try.
Also, keep in mind that for the price of a gps bike unit you probably can get a new cell phone with bluetooth and ant+, look that it has a good gps and battery life over camera quality or memory for instance, look that it is not very big and use it as a dedicated unit, but that would require research effort, and just getting a bike gps is more convenient.