Should I submit my XML sitemap using robots.txt or using Google Webmaster Tools?
Is only robots.txt submission is enough?
Using robots.txt to point to a sitemap for the most part is enough, however, I would suggest submitting it through Google Webmaster Tools as well. This does two things: one, it assures that Google is aware of your sitemap; and two, it will help speed up the process. If you have a Bing account, I would suggest submitting your site map there too. Bing has other methods too such as a ping option, but I cannot tell you how much trust can be put into any of them. You may want to create a Bing account if you want to submit a sitemap to them.
Once you submit your sitemap to Google, it will fetch the sitemap and use it for comparison. If your site is small or medium sized and Google can crawl your site with ease, then do not be surprised if Google prefers to crawl your site. Sitemaps, at the very least, will help Google to know that it can crawl your site properly and that is a very good thing. If Google finds pages specified in your sitemap that it cannot find by crawling your site, it will pick these up. No worries.