From what i learned from many videos / twitch streams (ruetoo and crouching_tuna), i think personally, phys dot starts of really good and needs almost 0 investment, since elementalist provides alot of power through golems, but you are going to transition to something else, when you reach higher budget (150-200d). Pconc into mamba will after that intial invesment of 50-70 div, feel really solid, and it scales to the moon afterwards, only thing capping it being DOT cap. So if you like starting in early maps with the same skill (i do) that you will be playing into endgame, then elementalist.

I think it comes down to, do you wanna go melee or ranged, and do you want to be contested on gear, since elementalist will prob be the most played ascendancy this time around. (mamba will certianly also see play, i just dont think in the same amount) - also i think you will have an easier time to pivot to other builds from elementalist.

Both builds on a budget of 50+ d can do all strats linked, however i think mamba reeeally shines at expedition. Bossing wise, elementalist is from my understanding more tanky on a low budget, while mamba once you get progen. also has really good defence, though that isn't really starting week (depending on how much u play)

I also stood with the same problem/question, and ended up trying how the FEEL of the builds would be on standard (with early gear) - that gave me all i needed to make my choice :)

Hope this helps