Atmega328p - PCB ISP pins for bootloader and sketch upload
Realistically, you should design ATmega boards to support both ISP and bootloader pins.  The logic is simple:


Unless you buy pre-programmed chips, or have a programming fixture, you will need ISP either to load the firmware itself, or to load a bootloader.  
Even if you plan to use only ISP for programming, it is still worth providing external connections to the UART pins for interacting with firmware during development; generally with ATmega-based Arduinos you do not have a breakpoint debugger, so debug messages on serial are your primary visibility into what is happening.
You should wire the reset line to somewhere that you can manipulate it, so you don't have to power cycle your firmware every time you want to restart it in testing.


Once you have done all that, you end up with support for both ISP and UART bootloader - regardless if you plan to use them both or not.