I don't have any experience with an externally mounted bow thruster, but I'd be concerned about durability. What happens if you hit a log? A piece of ice? Can it handle bumping into a mooring buoy all night? Or a bump against the dock?
I like my bow thruster...it makes docking faster, easier, and less stressful. Could I use a spring line instead? Sure, most of the time, but the bow thruster is quicker and easier. And spring lines can be tough to use when singlehanded.
What's the cost differential between a conventional tunnel thruster and an externally mounted unit? It'd be a bummer to buy the external unit, only to repair it, replace it, or install a conventional tunnel thruster in a short time.
On edit: Some boaters are anti-thruster for a variety of reasons—they enable people to "cheat" and avoid "real" seamanship, they're unreliable (not true in my experience), they're another thru hull, they give people false confidence, and on and on. Most of us use our boats for pleasure, and if a thruster (or thrusters) make boating more fun and less stressful and you can afford it, by all means get a thruster. Just know its limitations—run time (for electric), how much wind and current it can push against, etc.