It simply depends on which medium you are comfortable. Suppose you have a option to choose mode  offline or online.Then, go for counted number of practice test in both modes.

In my Opinion there can not be universal answer to this question and depends upon person in question.(The answer is written keeping IIT JEE mains-2018 in mind)

To keep it short i will analyse 3-main parameters to decide which one is more suited

1. XII Studying or XII passed (Preparation Level / Available time before exam ) — First factor for analysis is whether student is busy in boards or not.If student is giving XII boards exam then online exam (JEE Mains 2018) will provide one extra crucial week for revision/preparation.This extra time can work wonders if one utilizes this time effectively.But if your preparation level is very good and the extra time does not matter than you can opt for anyone of these two modes.
   Some people will argue that one extra week in mains is OK but there will be one week less for advanced examination —- I beg to differ as when one is going for mains exam then he will go through entire syllabus and will not tone down preparation level if he/she is seriously considering advance.On an average mains is easy no denying to this fact but certainly there will be some questions which will be equivalent (in some cases even tougher) than those questions which will be asked in advance exam
2. Only mains aspirants OR appearing for both mains and advanced —
   As we all know that JEE Advanced 2018 will be online only so JEE mains 2018 online examination provides a good opportunity to practice in real exam environment .
   Even if you have done tons of practice for online exam still nothing beats the experience of real exam when things are at stake and brain do have some pressure to perform . So if one is giving only mains then he should select the mode in which he is more comfortable but if one giving both examinations then online should be preferred.
3. Comfort Factor in using computer — Though in this age of internet and mobile it should not be factor but still if one is not comfortable in using computers and lack practice of online exam then he/she can go for offline mode and buy some time till advance.
   Even if this the case one should immediately start practicing test papers in online mode as in Advance 2018 there is no escape.
   
   There are many other factors like Technical Problems , Chances of mistakes while filling OMR sheet , stress on eyes , Relative Toughness of two modes ,Ability to see entire paper at once ,Space for rough work , ease of changing the answered option etc.