Sorry I did not see this thread when it was first posted since I had both cameras in question at the same time and did some comparisons. My comparisons are probably to late to be of use for the OP, but it may be of some interest to others.
Here in the US the TZ60 is labeled as ZS40 and the TZ70 is a ZS50. The ZS40/TZ60 was announced in early 2014 and was replaced in early 2015 by the ZS50/TZ70.
The sensor size remained the same but the pixel count was dropped from 18 to 12. In addition, the GPS was dropped but the newer ZS50 eledtronic view finder had a noticeable increase in resolution. The lens remained unchanged. The ZS50/TZ70 was replaced in early 2016 by the ZS60/TZ80. Both cameras that are the subject of this post are now discontinued but still can be bought new.
My original thought was to go with the ZS40 with the higher pixel count because I do a lot of cropping of my bird photos and I thought the added pixels may give a sharper end result. I really was not that interested in low light indoor photos at a higher ISO setting so the potential lower noise of the lower pixel count ZS50 did not offer an advantage to me. So with that in mind, I did some comparison shots.
The settings in each camera were the same, the photos were taken immediately after each other, and the cameras were on a tripod with a 2 second shutter delay. All but the last photo was reduced to a width of 1600 to meet the forum posting requirements. All of the photos on the left were taken with the ZS40/TZ60 and all of the photos on the right were taken with the ZS50/TZ70.
Here are the cameras at max wide angle zoom:
(Original sizes 4896 X 3672 and 4000 X3000)
Here are the cameras at about 25% zoom with a different scene and lighting:
(Original sizes 4896 X 3672 and 4000 X3000)
Here are the cameras at full zoom:
(Original sizes 4896 X 3672 and 4000 X3000)
The goose in the above full zoom photo is also in the first max wide angle photo. It is against the far shore line a little to the left of the two signs. This shows the zoom range of the lens on these cameras.
Here are the cameras at full zoom of a park sign to get an idea of the sharpness of the two.
(Original sizes 4896 X 3672 and 4000 X3000)
Here is a full close crop of the sign pictures to compare sharpness:
(Original crop sizes (unchanged) 540 X 628 and 442 X 516)
To me, there was very little difference in the image quality of the two cameras. This is especially true looking at the first four photos reduced in size to meet the forum requirements. Even looking at the original images full screen, the difference to me was very slight. I thought the ZS40/TZ60 was just ever so slightly sharper and theZS50/TZ70 had just slightly deeper color and contrast. The differences in sharpness and contrast between the two was so small that it was not an issue. Any slight sharpness improvement of the ZS40 would be wiped out hand holding.
Ultimately it came down to features when deciding on which camera to keep. I had no need for a slow startup, battery consuming GPS and went with the ZS50 due to the noticeably higher resolution electronic view finder. So between the two, my recommendation is the ZS50 unless the GPS is an important option. The quality of the image will be virtually the same with either camera.