Ethernet port or USB adapter?

from my knowledge i would say the ethernet connector is better, more reliable, usb connections over a certain length of cable  tend to lose some of it's connectivity read this somewhere not so long ago..

I use broadband internet for long a day but not face this problem.

If you use an USB3 or USB-C Ethernet adapter on a matching USB port, it will be just as fast as a "native" ethernet port.

A USB2 device or port would give roughly half the maximum possible speed, but still perfectly usable.

Some laptops use wireless normally.

Is USB 3.0 same as USB-C connection?

No difference.  Assuming that both the ethernet card and the usb adaptor are the same speed.  Whether cards or adaptors, they will have a speed listed on them that shows what their maximum speed is.  Early ones were 10 MB.  Recent ones are 100 MB.  Modern ones are 1000 MB (or 1 GB as they both mean the same thing).  Now they have ones appears that are 10 GB.  (I installed a couple last week for some server equipment.)  

If the Ethernet card and the USB adaptor are both 100 MB cards, they will run at the same speed.  If they are both 1 GB cards, they will run at the same speed.  If one is a 100 MB card and the other a 1 GB card, they would run at different speeds with the first being 10 times faster.

But there is no difference in a build Ethernet port and a USB adaptor rates for the same speed.

In practice not a lot. With an ethernet port you are guaranteed 10/100/1000Mbps (depending on the port). USB 2.0 speeds max out at 480Mbps, USB 3.0 ethernet are limited to 1000Mbps