For any laptop (be it ultrabook or not), leaving it on 24 hours is not the determining factor

It's what kind of workload it is on that matter. 24 hours idle, 24 hours bit-torrent (only stress HDD), 24 hours doing intense rendering (stretch CPU), generate vastly different amount of heat.
ultrabook, being thin, has less space to dissipate heat.
Generally, it is a good idea to get external cooling solution (eg. cooling pad, table fan ... ) if you game or doing anything that going to stretch CPU or GPU.
Asus
K46CB-WX060H, i5-3337U + Nvidia GT740M@RM2199[2kg], 2GB, 750GB, W8, 4-cell, OD
Dell
Vostro 5470, i5-4200U + Nvidia GT740M@RM2249, 4GB, 500GB(5400rpm), W8, 3-cell
Prices are RSP, negotiable.
Use the remaining of your budget to increase RAM (4G stick ~ RM100, 8G stick ~ RM200 (Asus). The 5470 only has 1 RAM slot, so you can only swap out the 4G with a 8G stick), extend warranty (Dell, to 3 years ~ RM300), get SSD (128G ~ RM300, 256G ~ RM600) to replace the HDD (keep as external HDD).
thx for your suggestion and advice~ watching movie is consider as doing intense rendering?