These are both in the future tenses.
Mopalit ko og saging
--would simply mean I will buy a banana, indicating he will buy it in the future maybe tomorrow or whenever
Magpalit ko og saging
--usually when people say this, it might means Future Present??? The person will buy banana not in some distant future but anytime on the same day or in the next moments.

It is also better to say it in FUTURE PRESENT TENSE like this
Mamalit ko og saging
Usually the word Magpalit is used by the Tagalogs meaning 'to exchange' its not very Visayan to use that word. Technically buying is another form of exchange.

But these days, Cebuanos in particular are not into the tenses. Others would simply use them interchangeably, I'd prefer to use 'Mopalit ko og saging'

Let me give you another example, please do take note that the Visayan language is full of shortcuts...
Maghimo pa ko og payag.
--- I will build a nipa hut.

As I have said shortcuts... it can also be said like this:
M'himo pa k'og payag.
But the language is Abugida based meaning all consonants need to be pronounced with a vowel(although not all the time). Which is why it can be said like this

Mohimo pa k'og payag.
Hence, saying Magpalit or Mopalit can be used interchangeably. But again better to use 'mopalit'