Hi,
Well, it is interesting. In fact the Nica option was going strong, but ultimately was abandoned due to concerns about geological instability (volcanoes, earthquakes).
Let's suppose that, for some reason, the canal is built via Nicaragua. It has many consequences, the first being the impossibility for the Panamanian independence movement to succeed (no american intervention), so today, after some revolts, Panama would still be a "departament" (state) of Colombia, perhaps with some autonomy in the context of a federal goverment.
I guess the following will happen:
1. Panama will be a prime haven for insurgents. In this ATL it will still be a remote, undeveloped part of Colombia, and a natural extension of the guerrilla activity from the Colombian Chocó region. Also, of course, a lot of coca plantantion, labs, etc.
2. The second largest city in Panama, Colón, won't exist! It was built as a terminus for the Canal. I guess that the population will be considerably less than it is today (from 3 million OTL to perhaps 1 million)
3. No Darien Gap: the Pan-American Highway will be completed. Today it is interrupted at Panama because the Pan goverment just doesn't want much contact with Colombia...but in this ATL they are in the same country. Which will also mean that, by this time, most of the Darien tropical forest would have dissappeared. Also, that you'll see much more inmigration to the US from South America, people coming by land all the way from Perú, Ecuador, etc.
4. No SouthCom in Panama, no Schools of the Americas in Panama, no American bases there, nothing. So the logistics of some interventions will have to change substantially.
And for Nicaragua,
1. Much more money!
2. All the American presence in Panama OTL goes to Nica ATL: SouthCom, bases, the works.
3. How will Sandino's revolt would have fared?
4. And the later Sandinistas? Probably they could never win.
Well, it is interesting: a whole country never exists, and another country gets a boost and more problems at the same time.
Finally, there is some ecological concerns with PAnama's Canal right now: species do move through it.