I am compelled to add more to my previous answer because of the ambiguity I noticed earlier in the framing of the question, as well as after going through the answers of other members now.
Challenges of Unionization in the Private Sector
Forming a trade union in the private sector is generally difficult because of the natural apathy of most employers towards unionization. Particularly when the industry is knowledge-based and the employability of the employees is also higher, as in the cases of I.T. and I.T.E.S., unionization comes to naught. After the advent of globalization, economies have become highly market-driven, demanding liberal privatization and more flexibility of employment. The political affiliation of every central trade union stands in the way of flexible employment for obvious reasons other than workers' interests. That is why the apathy of private sector employers towards unionization is always on the increase, and they try to scuttle every move in this direction.
Unionization in the Public Sector
In respect of the public sector, unionization is relatively easier since almost all the political parties have their labor wings, and their primacy is dependent upon which party is in power. No CEO of any public sector organization can act in the manner he likes in respect of trade union relations. Since public enterprise is mainly an effort of employment generation, the intake of employment will be relatively larger, and this would easily lead to unionization right from the inception level with the guidance of major central trade unions.
Functional Aspects of Unionization
Regarding functional aspects, important issues like wage revision and other service conditions would be taken care of by the central leaders who are very adept in the art of negotiations and highly influential in making policy decisions. So, plant-level union leaders are left with only day-to-day problems within the plant and keeping the membership intact.
In coercive activities like strikes and work stoppages, there is no difference.
The larger the membership of the trade union, irrespective of its sector, the more effective its strength and bargaining power will be because of its more seasoned and mature outside leadership.