Cambodia: September or October?

Hi there,
I'm flying into Bangkok on the 9th September and after visiting Northern Thailand I want to go to Cambodia and Vietnam before making my way south. I know that September and October are still the rainy season in Cambodia, but I was wondering if I'm better off going to Cambodia first, in late September, or after i've visited Vietnam, or if it will make any difference?
The main places I want to visit are Siem Reap and Phnom Penh (obviously I want to go to Angor Wat) and potentially go to the coast and see Sihanoukville and Kampot.
My friend is meeting me in Vietnam and is quite keen for me to go to Cambodia first so we can then go straight to the Thai Islands after, but if I do will I be caught short? I've heard it can get muddy and that Sihanoukville will be dead.
Thanks!

The later you go, the less rain you will have. But in reality a little rain shouldn't put you off, it'll stay warm. And not very muddy, a lot of roads have been built in recent years.

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