Hey, I'm pretty late with this, but will answer anyways. Selim III tried to modernise his military and form a strict organised army following the European model, but faced opposition from his own army and especially the Janissaries and the local clergy and in the end was deposed. Muhammad Ali managed to eliminate all opposition for his rule in Egypt, through exiling Omar Makram an Egyptian nobility leader who was one of the biggest reasons Muhammad Ali got position of Wali in the first place, but also the biggest threat to his rule later on due to Makram's rising popularity. And after the Massacre of the Mamluks in 1811 he had total rule over Egypt.

Were Selim and Muhammad Ali most differed in their approach was that Selim only tried to modernise the army through buying better weapons and equipment, better training, etc. Muhammad Ali saw the need for a full modernisation of the state. He trained a professional bureaucracy by sending gifted Egyptian citizens on scholarships to Europe. His land and agricultural reforms, heavy taxations of farmlands (and then nationalising said farmlands) and wakf properties returned a huge revenue.The state had a complete monopoly of the economy, producers of virtually any goods could only sell to the state, and the state would then resell the surplus in foreign markets for even more revenue.

He then used those funds on more Europeans scholarships in all sorts of fields, weapon factories to produce his own weapons and ammunition, built 2 shipyards, military hospitals, etc. He also tried to establish a textile industry which was not very successful. Also important to mention is Muhammad Ali's excellent relationship with France. He would send them gifts and even Egyptian artifacts at times and in turn get industrial and agricultural experts and officers to train Egyptians.

His first attempt at creating an army was very insuccesful, through Sudanese slave soldiers after his conquest of Sudan. In the end he was succesful through using Egyptian civilians, which he at first avoided doing both out of caution of an Egyptian uprising against him and to not decrease his avaliable workforce.

To summarise, Selim III sought to create a modern military through spending on better training and weaponry while at the same time facing tough opposition and during a time his empire needed far more comprehensive reforms as it was lagging behind quite a bit, so it would have ultimately been a fruitless endeavour. Muhammad Ali laid all the foundations for a strong nation by eliminating opposition, streamlinging the economy, better education to serve his reforms and by becoming entirely self sufficient. Long term endeavour and not at all easy but infinitely more succesful than Selim's approach.

I don't usually answer questions here, so sorry if my answer was not well structured or wasn't satisfactory.