Req packs

is it better to save up to buy gold and special packs or is it better to just go for silver and bronze?

Bronze then Silver then lastly finish gold packs. Doing heroic firefight warzone for the firefight commendations will grant handfuls of silver and gold packs, not that hard to do.
You do bronze packs to finish common unlocks, when you get 500 RP back it means you’re done and you can move onto Silvers.
Silvers would then only grant uncommon and rare unlocks. Silvers contain your core items for Halo 5 and ultra rare and legendary are a luxury you don’t really need unless you need Mantis/ Phaeton and Binary rifles.
By finishing silver packs and getting 3000 RP back, gold packs would then only give ultra rare and legendary.

This is quickest and most efficient way to do it, and you you have the option to invest real money into this if you want to.

Req packs have changed, its better to go silver gold then bronze.