The Eurolines pass will definitely be cheaper than the Interrail Global Pass. Even more so if you travel in the lower season. Moreover, with an Interrail Pass you are more than likely to incur additional costs, such as compulsory seat reservation on several trains or extra fees for the use of high-speed trains or other "special" trains. (However, there might still be advantages of train travel, over bus travel, as regards comfort and networks, but that's not the topic here ... ).
A pass is very nice as it gives you some flexibility. However, if you want to optimize or rather minimize the travel costs, you should try to book the different legs separately. Of course, you need to settle down your itinerary in advance. This means that you have to plan your itinerary, including places and dates, well in advance. If you plan and book ahead, this might be cheaper than traveling on a pass. This is even more true if you are among the less young people (26+). But as you can see, this means giving up some freedom and spontaneity.