IT WOULD be hard to imagine a more different duo than the civil servant front-runners for a seat at Governor Mr Chris Patten's side. One is short and reserved, a canny back-room operator with an instinctive distrust for radical change. The other is outgoing and publicity-hungry, never hesitating to throw caution to the winds. Which is why Mr Patten's selection of a new Chief Secretary - a process that will certainly preoccupy him this year - is being seen as a litmus test of the Governor's performance between now and 1997. It represents a critical choice between the voice of stability in Secretary for Education and Manpower Mr John Chan Cho-chak and that of radical reform represented by Constitutional Affairs chief Mr Michael Sze Cho-cheung. There will be no middle ground. The decision will pit Mr Patten's instincts against the wishes of his civil service. The Governor finds himself in this predicament because the only other two possible candidates have dropped by the way. Treasury Secretary Mr Yeung Kai-yin failed to make it on to the Executive Council last autumn, and now jokes of being shunted into the dead-end job of chairman of the Public Services Commission. Secretary for Economic Services Mrs Anson Chan Fang On-sang - repeatedly under fire from Beijing - is no longer seen as a credible contender to be Sir David Ford's successor. Superficially, it might seem almost as if there is no choice for a Governor so determined on pushing through his own point of view. Certainly that is the way it appeared when Typhoon Patten first hit the territory last summer. For Mr Sze is, beyond any shadow of a doubt, the civil servant closest in style and outlook to the Governor. THE first Hongkong official to meet Mr Patten following his appointment, the two men immediately developed a rapport. The Constitutional Affairs chief has gushed effusively - critics claim fawningly - about his boss since. ''Under this Governor, everything is OK,'' he once said. ''He's a decisive man and that's what I like about him.'' Certainly, Mr Sze has reaped many a benefit from his admiring stance, with a meteoric rise into the heart of Government House discussions, on to Exco and even to Washington to represent Hongkong at British Foreign Secretary Mr Douglas Hurd's side. But he has also more than proved his worth in the Governor's eyes, dreaming up the idea - from a chance glance at a copy of the census that landed on his desk - of massive new functional constituencies that lie at the heart of the Patten package. He has been constantly on the frontline defending the Governor's policies - while other civil servants stayed on the sidelines - even to the point of taking pot-shots at Mr Patten's critics like Sir Percy Cradock and Mr Lee Kuan Yew. All of which should make Mr Sze the obvious choice for Chief Secretary. The fact that Mr Sze loudly disclaims any interest in the post (''It's all a load of codswallop,'' he insisted) is scarcely an obstacle. But what is perhaps an insurmountable barrier is the attitude of the officials he would have to lead, with some administrative officers already jealous of Mr Sze's rapid rise, calling him by a Cantonese phrase he interprets as meaning foolhardy (sam hau se goh yung ji, literally ''wears the word brave written across his chest''). Instead it is Mr Chan who would undoubtedly be the overwhelming choice of his fellow officials, were democracy suddenly to spread to Lower Albert Road. That is partly because Mr Chan, the first local Chinese to be appointed to a senior Government post, has made remarkably few enemies in his years of grooming for the top - shuffling between key jobs, building up experience while avoiding the kind of controversy that has afflicted his rivals. But, more crucially, Mr Chan is seen as a symbol of stability at a time of great uncertainty, with China even threatening to purge the civil service in 1997. He is respected enough in Beijing to stand some chance of stalling such attacks. He may not have as much presence as his rivals; he may not even have shone of late - particularly during last year's labour import controversy. SUCH drawbacks, however, are outweighed by his reliable appearance - and perhaps even his differences from the Governor - in the eyes of some officials who six months ago thought differently. While Mr Chan has loyally muttered a few words of support for the Patten package, he does not deny these were out of duty to support Government policies rather than any sense of conviction. His beliefs may well be better represented by the white paper he pushed through in 1987, which put the brakes on a faster pace of democratisation. All of which might seem hardly a recommendation for the Governor to choose him as the next Chief Secretary - save for the preference of politicians worldwide for an opposing personality as their deputy. So far, he has apparently accepted every appointment suggested to him by the Chief Secretary. With Sir David more than likely to recommend Mr Chan - who he has worked closely with over many years - as his successor, the Governor can be expected to endorse that choice. Ironically, it may well turn out Mr Chan's differences from Mr Patten will prove to be his stepping stone to the top.