I think the answer to this is BOTH, but it depends what you mean by a 'religion' and what you mean by 'philosophy of life'.
It is a religion because it involves the elements of belief, faith and self- transformation, which do belong to some philosophies as such but which I believe are more characteristic features of religions. In all religions I would argue that faith and belief are paramount and also the self-transforming aspect. People turn to religion very largely because they thought it can improve their life, their self or their moral conduct or to take greater comfort in life. This is why religion is termed the 'opium of the masses' it is a 'bolster' which "protects" ppl from the bad times. At the very least, it offers that to us in the form of 'understanding which gives comfort'. In this sense a religion is a set of beliefs which we take comfort from but which cannot be conclusively proved objectively as true or false. So Buddhism is religion too