1) A registered gift deed is the best option as the property will be transferred in to your name during the life time of your father.
2) Upon execution of Gift deed, you can mutate the government records such as property tax, electricity bills, water bills etc in your name and thereby become the absolute owner of the property.
3) Once the property is transferred in to your name, you can also mortgage the property and thereby avail loan facility from the bank.
4) A WILL shall become effective only after the life time of your father.
5) Also a WILL needs to be probated in the court of law
6) The Stamp Duty and Registration Charges for both WILL and registered Gift deed are almost same (Stamp duty and registration charges for a Registered Gift deed will be marginally higher by about 0.05%).
7) Your son will not have any rights on the property once the property is Gifted to you by your father. Please note that Hindu succession law has amended the erstwhile hindu laws of succession. Son of a living son cannot claim a share of his grand father's property vide section 8 of Hindu succession act
8) Conditional Gifts: . The position of law is that once the gift is made and the donee(you) accept the Gift, the conditions will cease to exist. Also a conditional gift deed will not be of any use to you as you can neither enjoy the ownership of the building or avail loan based on the conditional gift deed.
9) Better to go for a Registered Gift deed now and complete the transfer.
Hope this helps