Rather than shows preference. This expression is generally used in 'parallel' structures. e.g - with two nouns, adjectives, adverbs, infinitives or -ing forms.
e.g.
1). We ought to invest in machinery rather than buildings.
2). I prefer starting early rather than leaving things to the last minute.
When the main clause has a to - infinitive, rather than is usually followed by an infinitive without to or -ing form.
e.g. - I decided to write rather than phone/phoning.
Instead of suggests that one person, thing or action replaces another. Instead is not used alone as a preposition; we use the two words instead of.
Instead of is not usually followed by an infinitive.
e.g.
1). I'll have tea instead of coffee, please.
2). I stayed in bed all day instead of going to work.
3). Amit was invited to the reception, but he was ill, so Akash went instead of him.
Note: Instead (without of) is an adverb. It begins or ends a clause usually.
e.g. She didn't go to Greece after all. Instead, she went to America.
Usage: instead of + noun phrase. Instead of is only a preposition and can introduce only a phrase i.e no verb.
Usage: rather than + verb (or) rather than + noun. Further rather than can act as a preposition and can introduce a prepositional phrase or can act as a conjunction and introduce a clause
Mr. EGG explains the usage in simple way via cartoon on the link below:
rather than usage
To understand the use of the above concept view questions 22 and 23 by clicking on the link below.
http://gmatsentencecorrection.blogspot.com/