With the 170 piece you gain the highlighted parts in red below.
3PC Rachet Handle: 1/4, 3/8, 1/2
10PC 1/4" Drive Regular Socket mm: 4, 5, 6, 7, 8, 9, 10, 11, 12, 13mm
10PC 1/4" Drive Regular Socket SAE: 5/32, 3/16, 7/32, 1/4, 9/32, 5/16, 11/32, 3/8, 7/16, 1/2
6PC 1/4" Drive Deep Socket mm: 4, 5, 6, 7, 8, 9mm
1PC 1/4" Drive Wobble Extension Bar: 100mm
11PC 3/8" Drive Regular Socket mm: 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19mm
9PC 3/8" Drive Regular Socket SAE: 3/8, 7/16, 1/2, 9/16, 5/8, 11/16, 3/4, 13/16, 7/8
8PC 3/8" Drive Deep Socket mm: 10, 11, 12, 13, 14, 15, 17, 19mm
2PC 3/8" Drive Wobble Extension Bar: 75mm, 150mm
1PC 3/8" Drive Breaker Bar (new type) 200mm
7PC 1/2" Drive Regular Socket mm: 20, 21, 22, 24, 27, 30, 32mm
4PC 1/2" Drive Regular Socket SAE: 15/16, 1, 1-1/16, 1-1/4
4PC 1/2" Drive Deep Socket mm: 20, 21, 22, 24mm
7PC 1/2" Drive Impact Socket: 13, 14, 15, 17, 19, 21, 23mm
1PC 1/2" Drive Wobble Extension Bar 125mm
1PC 1/4" Drive Spinner Handle
8PC E-Torx Socket: 8, 10, 11, 12, 14, 16, 18, 20mm
2PC Spark Plug Socket: 10, 14mm
6PC Accessories 3/8" UJ, 1/2" UJ, 1/4" Bits Adaptor, 1/4"Fx3/8"M, 3/8"Fx1/4"M, 3/8"Fx1/2"M Adaptor
30PC Screwdriver Bit: Slotted 4, 5.5, 6.5, Hex 3, 4, 5, 6, 7, 8, 10, Phillips 1, 2, 3, Pozi 1, 2, 3,Torx T10, 15, 20, 25, 27, 30, 40, Tamper Torx T10, 15, 20, 25, 27, 30, 40
20PC Bit Socket: TX-10, TX-20, TX-30, TX-40, TX-45, TX-50, TX-55, TX-60, HW-3, HW-4, HW-5, HW-6, HW-8, HW-10, Phillips 1, 2, 3, Slotted 4, 5.5, 6.5
10PC Hex Key: 1.5, 2, 2.5, 3, 4, 5, 5.5, 6, 8, 10mm
9PC Ratcheting Combination Wrench: 8, 10, 12, 13, 14, 15, 16, 17, 19mm
And lose:
8PC Combination Wrench 8, 10, 12, 13, 14, 15, 17, 19mm*
3PC Double Box Ratcheting Wrench 8x10, 12x13, 17x19mm*
Screwdriver Bit Slotted 4.5, 6
* In place of these you get the 9PC Ratcheting Combination Wrenches instead.
So it depends if you value the extra bits above in the 170PC set for an extra £24.99 over the 150PC. If I was buying right now my first good socket set, I'd probably get the 170PC. In my experience you can never have enough tools and you will always end up using the extra bits at some point. Be warned, though as said above already, there will always be jobs you come across where despite owning one of the above sets, you will still need to go out and buy a specific "odd" sized, or larger sized specific socket.
I prefer the older version of the 150PC that I own where the bits are not inside that pull out tub. I like them all presented open in the main tool case, but it's not a big deal.
I rate the Halford Pro stuff. Never had a problem getting a couple of bits replaced either. Even the ratchets, which some say should not be covered as it is a moving part. I just wondered in without a receipt and it got swapped there and then for my old. Easy. On a Sunday too. Very handy when in the middle of a big job.