Noob her!
why use a 6502 versus another 8bit cpu like Atmel AVR?
Is there any advantage?
Does it consume less electricity while running at similar speed ?
I'll step in and say that the Atmel AVR is damn awesome processor, it's just that it's hooked up to on-chip peripherals and the address/data bus is completely internal. This means that if you want to connect something to an Atmel AVR, it will likely be over some serial protocol, like SPI, I2C, or UART.
This is what you want, like, 99.9% of the time.
The 6502 just puts its address and data bus on pins, and you can make the memory map for your computer however you want, as long as you are willing to wire up the logic. There's no privilege for on-chip resources... everything is off-chip and completely equal. With the AVR, the resources (RAM, flash, peripherals) are completely fixed but much, much faster. There are a ton of chips you can buy with SPI / I2C interfaces if you want the AVR to do something interesting, and you drive them with the on-chip peripherals. The on-chip peripherals get to sit in memory somewhere, but the external ICs don't. With the 6502, you put things in memory somewhere, figure out a way to decode addresses on the bus, and use a parallel interface for everything.
One thing that might or might not matter, depending on why you are doing the project, is that a 6502-based computer is less opaque.
A 6502 is a CPU in the "traditional sense". It has limited internal state in the form of a few registers, and communicates with the outside world with address lines and data lines; I only simplified a little there. Everything else is external: RAM, ROM, any external devices on a different bus (I2C etc.), even timers and GPIO need external circuitry with usually other ICs.
An AVR on the other hand tends to have all of that stuff (or at least a sizable subset) built in. That's, obviously, enormously convenient, and often means that with the AVR itself and just minimal external circuitry you have your needs covered. But it's less educational, you've rather plugged a computer into your circuit than built a computer.
Sentimentality. The 6502 has been around for almost half a century now. Often the processor in an older programmer's first computer or video game system. It's horribly limited, but assembly programming for the 6502 has become a kind of code golf.
6502 is about as simple a CPU as you can get without it being a complete pain in the ass to work with. So it's a great CPU to learn nuts and bolts of computer hardware. It's pretty easy to interface it to external devices, pretty easy to learn the instruction set, there's a huge community of enthusiasts for it, it's very cycle efficient and very responsive to interrupts and somehow feels fast, and the ins and outs of how it functions are super well documented and reverse engineered.
The Atmel has a modified harvard architecture, so it's easier to use as a microcontroller than to run an OS on. Where there are lots of available OSes for 6502, Z80, etc.
More fun/interesting and more potential to be actual computer.
Probably the main advantage of a 6502 is more RAM/EEPROM (as 8bit microcontrollers are pretty limited that way) and more of a hardware/software ecosystem to work with.
This is what you want, like, 99.9% of the time.
The 6502 just puts its address and data bus on pins, and you can make the memory map for your computer however you want, as long as you are willing to wire up the logic. There's no privilege for on-chip resources... everything is off-chip and completely equal. With the AVR, the resources (RAM, flash, peripherals) are completely fixed but much, much faster. There are a ton of chips you can buy with SPI / I2C interfaces if you want the AVR to do something interesting, and you drive them with the on-chip peripherals. The on-chip peripherals get to sit in memory somewhere, but the external ICs don't. With the 6502, you put things in memory somewhere, figure out a way to decode addresses on the bus, and use a parallel interface for everything.