Steve,
It's an old adage when powering a boat that you'l never regret extra power...
However, with an eletric on a personal pontoon, that isn't exactly true.
30# thrust is plenty for your boat. I use one that size to push around my 15 1/2 foot aluminum bass baot, and it works just fine. With the light weight of your boat such a motor will move you along right quick.
There are other considerations, though. Battery weight, as Jeremy pointed out, can be an issue. I have an 8' pontoon, and the 30# motor and a size 24 marine battery from my boat power and balance it well. But I'm a big guy, around 250, so the fifty pounds or so of battery and motor placed behind me don't make the boat feel awkward. If you are a little fella, that extra weight behnd you may have you looking up all day...You can usually change the tube/deck/seat placement on most 'toons to help balance this out.
Battery LIFE hasn't come up yet. This is the main argument for a smaller thrust motor. A 50# thrust motor will use up the battery much faster than a 30# thrust one will. You'll not likely be carrying an extra battery on your 'toon, so this can be an issue if you like to fish for longer than just a couple of hours. As an example, at full thrust my 30# motor can go for around three hours on a fully charged #24 marine deep cycle battery. If you use a lower speed, the battery life increases dramatically, and at 'low' speed setting it will run for over ten hours. Conversely, the 48# thrust foot controled motor on the bow of my bass boat will only run a bit more than one hour at high on a fully charged battery. (one reason I carry two batteries in the boat).
You should be fine with a 28 to 30 pound thrust motor and a regular 12v marine battery. You have a couple of size options, get the biggest one you feel will fit on your boat. If you have the funds, you can get dry cell batteries that are very light, but they are expensive.
I went for a few years 'rowing' my 'toon about. Fine on a small pond or if you aren't going far on a lake. But if you get caught out in some wind, or if it's typically windy where you fish, or you want to travel more than a few hundred yards, an electric motor will make your fishing more enjoyable.
And, don't forget to include an ANCHOR. It can save your life as well as make it easier to fish in any kind of breeze...
Buddy
It Just Doesn't Matter....