If Mr. Woodhouse had been concerned that John and Isabella had gone to a less fashinable place, he would have recommended that they went to Bath (you know, that place where baronets and dowager countesses went ;-) ). As Robbin has pointed, the only reason that he prefers another place is that Mr. Perry has gone there, and in fact his preference is for the *less fashionable* place.

According to The Cambridge Editions of the Works of Jane Austen (emphasis mine) :

South End: now Southend, a town in Essex, on the Thames estuary, forty-two miles from London. It was only in the 1790s, with the construction of a terrace and a new hotel, that it became a seaside resort. It became fashionable after a visit in 1801 by Princess Charlotte, and in 1803 by her mother, Queen Caroline. By 1803 its population had grown to 2000. JA's brother, Charles, visited Southend with his family in July 1813 (3-6 July 1813, L, p. 216). The Guide to All the Watering and Sea-Bathing Places (London: Longman, Hurst, Rees, Orme and Brown, 1815) notes that the company is 'mostly of the superior rank of society, the lower orders of the community not having as yet intruded themselves into Southend, as into many other places of this description'.