I never handled a K-S2, but I have a K30 and the DA18-135WR, and I enjoy both, and dont feel the need for a camera upgrade, whereas I regularly buy extra lenses (my last buy was in march, a 2nd hand DA70 f2.4 limited).
I suggest you go to PentaxForums, where you will find the most complete database on all Pentax gear, with in-depth staff reviews and many user's reviews of K30, K-S2 and DA 18-135 WR:
IMO, the K-S2 is in the same league as the K30. Its shutter and EVF should be the same. They both use the same Li battery (but with the K30 you can buy an extra adapter for using AA batteries).
The 20MP vs 16MP should not really make a great difference.
Whereas the suppression of the AA filter should really improve the capacity of the sensor to extract more details, therefore, with a sharp lens, the K-S2 should deliver sharper pictures than the K30, like K5IIs vs K5II.
But with an entry level kit zoom, there may be no visible difference.
The most sensible difference should be the in-camera wifi effective remote control and the swivel screen, two very convenient additions.
According to reviews and Ricoh documentation, the K-S2 AF seems very close to the K5-IIs AF, which means a better AF capacity than the K30 in very low light, but no sensible improvement in tracking performance (for that, you need the K3, but it is a much more expensive pro-level camera).
The Pentaxforums in-depth staff reviews will give you much more details on both K-S2 and K30.
The K30 with DA 18-135WR is a very convenient kit, and you should get it at a good price because the K30 is a 2012 camera, replaced in 2013 by the K50 (which is quite the same camera in a restyled body), itself replaced now by K-S2.
I never had any issue with my K30, nor the DA 18-135WR.
The DA 18-135 is a superzoom, so there are compromises in sharpness and aberrations.
Which means, if you are a pixel-peeper, you wont like it.
But if, like me, you appreciate the convenience of a all-in-one compact WR lens, not having to change lens on field and yet being able to handle most scenes, with the typical Pentax high contrast and excellent colour rendering, you will love it.
For travel , you can use it like a point and shoot, and it delivers (see some holidays pictures on my flickr account or my DPR gallery). And it is WR!
Aberrations can easily be corrected either automatically in-camera if you shoot jpeg, or by using the lens profile in LR5 or DXO if you shoot raw.
The 18-135 is very sharp in the center, and rather resistent to flare. The border and corners are soft wide open at all focal lengths, and are still soft stopped down above 70mm.
It is not the best lens for architecture, but, in most real life scenes, the border softness, while not pleasant at pixel level, is not really visible if you look your photos on a HD 2MP screen, or for prints up to A4 (=8"x12").
And the K30 sensor can give you much sharper images if you mount a good prime lens, for instance one of the two very affordable "plastic wonders" DA 50 f1.8 or DA35 f2.4, and is good enough to get oustanding sharpness and IQ with any of the Pentax limited or DA* lenses.
Of course this does not mean that you cannot get still sharper images or better IQ by using a K-S2 or K3, what I say is that the K30 is a very good and responsive camera, and, as with all interchangeable lens cameras, the camera is only part of the system.
Tatouzou,
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