Grammar Notes Tradtional vs. Current

TRADITIONAL GRAMMAR	LINGUISTIC DESCRIPTION
In traditional grammar, than is a conjunction used in comparatives with adjectives and adverbs.  Than introduces the comparative clause. HIstorically, "rath" meant "soon" (adv.)   – I'd sooner / rather have this than that.  The expresson rather than coordinates two syntactically alike items in a statement of comparison.	In linugistic description, rather than (X to avoid Y) is a preposition with a clause as its complement.  The clause usually has a base verb form (bare infinitive), but may also have a gerund verb form.
(X not Y) We walked home rather than drove home.  (adverb + conjunction pair that forms a comparative expression)	(X not Y) We walked home rather than drove home.  (a coordinator in a comparative expression with a parallel complement type)
X to avoid Y We walked home rather than sit in traffic.  (same as above)	X to avoid Y We walked home rather than sit / sitting in traffic.   (a preposition with a gerund complement) Rather than sitting in traffic, we walked home.

Than is a preposition (not an adverb) that accepts a wide range of complements. (A number of words that were previously analyzed as adverbs are now analyzed as prepositions.) (CaGEL 612) ; NP –noun phrase; N – noun; VP – verb phrase; V – verb; Comp – complement; Coord – coordinator; Det – determainer; PP – prepositional phrase; P – preposition; Sub – Subordinator

Complement Types of  Prepositions: "instead of" and "rather than:

INSTEAD OF	COMPLEMENT TYPE	RATHER THAN
He took the laptop instead of the iPad.	NP  (noun phrase)	He took the laptop rather than the iPad.
I'll go instead of you.	noun	I'll go rather than you.
We'll go now instead of in the morning.	PP	We'll go nowrather than in the morning.
I felt humiliated instead of proud of my self.	AdjP	I felt humiliated rather than proud of my self.
He spoke timidly instead of competently.	AdvjP	He spoke timidly rather than competently.
We want them to be bold instead of be cautious.	bare infinitival	*We want them to be bold rather than be cautious.
*It's better to continue instead of  (to) wait.	infinitival	It's better to continue rather than (to) wait.
We prefer moving on instead of waiting.	gerund-participle	We prefer moving on rather than waiting.
He said that "it" was regretful instead of that he was sorry.	declarative clause	He said that "it" was regretful rather than that he was sorry.
They told me I had tried hard instead of whether I had succeeded.	closed interrogative	They told me I had tried hard rather than whether I had succeeded.
They told me where I had to go instead of when I had to go.	open interrogative	They told me where I had to go rather thanwhen I had to go.
He asked that they be heard instead of that they be sent away.	subjunctive clause	He asked that they be heard rather than that they be sent away.

"The prototypical PP has the form of a preposition as head and a NP as complement… Prepositions allow a wide range of complement type." (CaGEL 642) 

Notable Comments

Rather "contains the comparative suffix -er but the original base rather (meaning "soon") has been lost, so that rather  is no longer analysable as an inflectional comparative.  It nevertheless retains clear semantic and syntactic affinities with ordinary comparative constructions." (CaGEL 1128) 
Rather than – I'd rather resign than accept such humiliation.  "Here it is an adverb with a comparative meaning: approximately "more readily, in preference to".  There are also uses where this meaning is largely or wholly lost — a change facilitated by the fact that the morphological base rath- no longer occurs without the -er suffix." (CaGEL 1317)
Than —  " The most usual position for the comparative is at the end of the clause containing the comparative phrase…" (1106);     "Bob is more generous than Liz" — Liz can be regarded as a "reduced clause" or as an "immediate complement NP" (CaGEL 113)

Conjunction or Preposition   "The question that puzzled Fowler was whether rather than always operated as a conjunction and thus had the same construction before as it had after, or whether it could also operate as a preposition and so connect dissimilar constructions…. We will simply point that rather than does function like a preposition. 
'Rather than argue for the overthrow of the entire system, the Colonists realized...that the basic values of British law were still valid. —  Daniel Sisson.
But when parallel constructions appear on each side of rather than, it is functioning like a conjunction:
" ...implicating them, this time subtly rather than powerfully — J. I. M.Stewart"
(MWDEU 797)
"Than is both a conjunction and a preposition…. In current usage than is more often a conjunction than a preposition... me after the preposition is more common in speech than in edited prose." (MWDEU 892)