i am using merida 400 road bike. using this byke both for triathlons and long rides
Since you are planning to use the same aerobar on your road bike for triathlons and endurance rides. You need to keep in mind that you will need an aerobar with risers.
The triathlon position will be too aggressive for endurance and vice versa. The purpose of the two are entirely different. In a TT setup, being aero and fast is your primary concern, while for endurance comfort is your biggest priority.
Generally for a bikepacking setup, the aerobars are kept slightly higher than your saddle.
For your usage, alloy bars will be more than sufficient, the added weight saving of carbon is not required.
You can additionally choose to adjust the entire cockpit height with the handlebar stem risers. Higher when you are planning endurance rides, and lower when you are racing.
This adjustment will of course effect your entire bike fit, which means you will need to adjust the saddle fore-aft position also when switching between the two riding styles.
When it comes to buying an aerobar, the more angled bars will be more comfortable on your wrists, but less aero.
Considering you are using the same bars for racing and endurance, I would suggest going for comfort.
You can have a look at these two from BOTS and Mastermind respectively:
PRO Bike Gear Missile Ski-bend Clip-on Aero Handlebar