First of all, all Indian music originates from Dhrupad. This was the original temple singing style prevalent at the time. Khayal is a relatively recent development, but draws on Dhrupad for its stylistic content. In fact many of the recent khayal singers were quite well versed and deeply rooted in Dhrupd. The Agra gharana i a good example. Dhrupad consists of the extended Nom Tom alaap, followed by compositions set to taals on the pakhawaj. The alaap in Dhrupad can appear to the casual listener as discrete distinctly separate notes, although that is a complete falacy. Khayal on the other hand appears to the casual listener as having a lot more "continuity" of notes. Dhrupad used to be quite rigid in the rules regarding presentation. Khayal came into existence as a "looser" version of Dhrupad. Many volumes can be written about these two styles, but let us move onto the bansuri.
The Dhrupad style bansuri vaadan is more in line with the vocal style of Dhrupad, i.e. after the alaap, which is more or less free form, setting up a rhythym with breath control (jod), and moving onto to more rapid rhythymic excursions (jhala). This is then followed by compositions set to various taals. The gayaki ang bansuri is very similar in the alaap part, but then directly moves on to compositions based on various taals, exactly as in Khayal singing. i.e. after the alaap, the vilambit composition, then the madhyalay and then the dhrut (or a combo thereof). The gayaki ang does not go into the jod/jhala formats, but involves a lot of taankari.
Having said that, both forms of bansuri vaadan require the same level of training in terms of breath control, tonguing, blowing etc.
Hariji's style is more of the Dhrupad style (although he is quite adept at any style), while Pannalal Ghosh's style was more of the gayaki style.
Hope this answers your question. More on this should you require more information