Re: what is better?internal or external oscillator?enable or disable MCLR pin ?
Any time the manufacturing offers an option, this is because in some situations one choice is more suitable, but in other situation you may need a different choice.
Fore alarm system, you probably don't need an external crystal and internal oscillator will do just fine, but if you need to maintain time, the internal oscillator will not be enough, thus you need something, such as RTCC.
It's a good idea to enable MCLR and not mess with it unless you're running out of pins.
Speaking of choices - selecting an obsolete microcontroller for new designs is rarely a good choice. There are many new PIC18s, and also newer PIC16F1*. These will be much better choice.