I saw KWEB on Arca and LSEetf in ibkr. Does it mean Kweb also listed on Lse??? Which is better to buy?

Is there an advantage in knowing the closing price of KWEB in the USA market and thus the likely opening price in the UK?

Hmm, KWEB's dividends fluctuate quite a bit, but from what I can see, their last dividend of 22c, was a 0.36% yield. A 15% savings on dividend WHT would represent a 0.054% savings from going from NYSE to KWEB.

Note that KWEB on LSE has a 0.75% expense ratio vs NYSE's KWEB which has an expense ratio of 0.73%.

So your net savings, will be around 0.034% (0.054% - 0.02%).

BUT

IBKR's commission structure is different for LSE and US exchanges. IBKR only charges around USD0.35 for US trades while charging USD5 on the LSE. As such, I would actually think that KWEB on NYSE poses a better deal on IBKR. Moreover, since dividends fluctuate quite a bit on KWEB, the dividend savings might be reduced as well, making it less worth it. For example, while 2020's dividend was 22c, it was only 4.1c in 2019.