In classical music notation you see a bunch more cres. and dim. than you see in film score notation.  Film score notation is almost purely hair pin dynamic changing.   Can they be used interchangeably? Sure, sure they can but I think it is best to use them more cleverly. 

How I use hairpins and cres. and dim.

I have a very specific idea of how to use these dynamic modifiers in music.  If I want a specific length cresendo, starting and ending at a certain point I use a hair pin.

If I want to leave it up to the performer’s instinct I use the word.  

So basically the concept is; If I am going to be really specific about it I use hairpins – if I want to give some liberty to the performer to phrase it in their own I use the words.

If you want to have a generally increasing line with some give or take you should use both.

Refining this concept further: 

A) Since hairpins are the mainstay of studio players I’ve found that if I do write “cres.” and “dim.” sometimes they are ignored on the first pass.  If I remind the players to consider these text modifiers then everything is fine.  I’ve noted that some places outside of LA and London simply don’t read dynamics – that is something to consider as well.

B) Since hairpins are more exact they tend to work better for quick dynamic changes where multiple persons are involved. (like in a large section)

C) Text modifiers seem to work much better for solos and expressive passages.  As I get older and more confident as a composer I find myself wanting to hear how some of these excellent orchestral musicians can contribute to my work – this is an easy way to allow it.  

D) Text modifiers allows your conductor to be involved. There are several excellent classical conductors in Los Angeles who also conduct sessions. They are capable of shaping things in a very musical way if you leave them some room to function.  They can do more than wave a stick to click if they are given room to do so.

Rallentando vs Ritardando (ps this is just my opinion)

These words are used more or less interchangeably by most music folks these days.  They both imply a slowing down of the tempo but is there any more to it?

Personally I tend to think of Rallentando as a “holing back” of the tempo whereas Ritardando can be more of a linear tempo change.   Rall. to my brain seems to have more awareness of the tempo it is coming from whereas rit. seems to be less interested in the previous tempo, but more interested in the slow down.

Additionally sometimes I write “holding back” as a direction – I feel it is an easily accessible to conductors who can literally feel the orchestra fighting their tempo change.