Keep them coins. Arachnid isn't worth the coins, No backup isn't as useful as some other titan exotics.
UR for 13 coins is probably worth it if you have coins to spare and actually want to use a shotgun as a primary.
UR is 17 coins, I have 14 right now. I'll probably save it as I will most probably get it as a drop someday, armour exotic drops are harder.