Which laptop is better and why?

Both nearly identical and inferior value to:

http://www.currys.co.uk/...teria.html
£399 vs £499
HP Pavilion 15.6" Laptop in choice of colors while on sale
Every laptop is a set of parts.
Intel® Core™ i5-4288U (4524 score) vs i7-4510U (3985 score)
Memory: 8 GB same
Hard drive: 1.5 TB (bigger)
Graphics Intel Iris 5100 vs Intel HD 4400 - gaming/graphics much higher on the HP

Lenovo G series is their Essentials line, and no notable quality difference among Essentials, Acer, and HP Pavilion.

HP has only 10/100 ethernet so hardware ethernet cord connected is a slower download and streaming by a bit.
The i5 is higher wattage as 28 vs 13, so Acer probably gets better battery life, Lenovo 2nd, HP is a bit lower.
HP is a little lighter in weight.

Now, I don't like HP, but don't like Lenovo G or Acer either, and since you seem to value performance over quality of build at a price point, might as well buy the best value.

Difference from the Lenovo to Acer is insignificant. Acer has a better battery life, Lenovo a bit better keyboard most likely.

The HP is cheaper, faster, better graphics processor, lighter in weight, bigger HDD, 4 color choices.

Thinkpad Edge if you want higher quality
http://www.saveonlaptops.co.uk/20C600JBUK-Lenovo-ThinkPad-Edge-E540_1651795.html
The i5 speed is between the two cpus, and graphics between the two graphics performance.
You get some nice features in a Thinkpad.