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Is the philips 7134 based tuners that much better...
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This is based on a misunderstanding. Neither the CX23881 nor the Philips SAA7134 are tuners. These chips simply accept a video signal and digitize it. This signal can come from an outside video source or from a TV tuner that is soldered on the card, but for the chip this does not make a difference. The confusion probably comes from the fact that Philips also manufacturers most of the tuners used on TV cards.
The only specific TV related function these chips do have is the ability to decode (stereo) sound from the tuners video signal. This is why you often find two versions of these chips - CX23881 and CX23883. Both can process NTSC , SECAM and PAL video, but only the CX23883 can decode the stereo sound encoded in American TV broadcasts.
So, depending on the brand of the card, you will find the CX23881 and SAA7134 teamed with different brands of and different quality tuners. There are probably CX23881 based cards around with Philips tuners on them too.
It has been my experience that TV card tuners are less tolerant for weak antenna signals then the ones in TVset's or VCR's, but provided the antenna signal is good the quality of TV card tuners is equal or sometimes even exceeds what you find in consumer video equipment.
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If you have a good VCR with S-Video out (or S-Video out on your cable reciever) you're a lot better off avoiding the card's tuner completely by using the VCR as your tuner and running the S-Video line to your capture card's S-Video in.
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This is partly true. TV broadcast is by its nature a composite video source. To digitize it, chroma and luma has to be separated by a combfilter. Both the CX23881 and the SAA7134 incorporate excellent digital comb filters.
Using an external tuner with s-video out only has an advantage if both the tuner is better AND the VCR has the better combfilter of the two.
For example, My JVC HR-S7950 s-VHS deck has excellent playback capabilities, but it's tuner is only passable in quality.