How does 衣衫 compare to 衣服 in terms of meaning and flavour?
In olden time, there might be difference between the two terms

「衣 」[ (2) [upper garment;jacket]
衣,所以蔽体者也。上曰衣,下曰裳。]

and 

「衫」, [(1) 古代指无袖头的开衩上衣 [sleeveless jacket with vent in the sides of a jacket]. ]

In modern Chinese language, both 衣 and 衫 simply refers to 'clothes one wears on his upper body'

Together, 「衣衫 」is a compound word for the general term 'upper garment' much like 「刀劍 」(knife and sword) is a general term for 'weapons'

「服」as a noun, refers 'every article of clothing one wear on his person'. For example:  「軍服」(military uniform), 「制服」(uniform). 

「服」is also a verb for 'to wear' 

Together,「衣服」is a general term for 'clothes/clothing' just like 「刀斧」(knife and axe) is a general term for 'weapons'


  「衣服」 is much more common than 「衣衫」 in day-to-day words.


A related term is 「衣裳」(upper garment + lower garment), which is also a general term for 'clothing'