Marriage is one of the most important events in every body's life that requires special attention. The principles and conditions that are necessary and conducive to consolidation of family are of two types:
It should be noted that it is difficult to know an individual's personality and character in one or two sessions because some people cleverly hide their weak points and deficiencies and show themselves as having faith and good moral character so as to attract the attention of chaste and honest people. It is therefore necessary that in order for a girl not to fall in such people's trap she or her family should conduct serious and comprehensive research into the lives of such people in their families and work places and to enquire their friends and neighbors about their moral character.
2. Personal features:
Every individual has his own personal features and traits such as the way he dresses, his complexion, his manner towards others etc. which are important and should not be overlooked because the greater the harmony and compatibility, the stronger would be the foundation of the family.
In the light of the above criteria which were explained briefly, it seems that the best way to choose one of the two individuals is to see which one is more qualified and matches with the above criteria. You can give them points for each of the criterion and whoever scores more points is apparently more suitable for marriage. It should also be noted that good deeds are not all in the same level and they do not get the same point. For example, Islam attaches special importance to prayers and if a person does not offer the prayers and bears other good virtues, he will not get any point for those virtues. Faith and adherence to religious issues and also good morals which are part of the doctrines of our religion are more important as compared to other features and characteristics.
Note: What do "irritability" and "extremity" mean? If a person gets angry seeing someone abandoning obligatory duties and doing haram acts and he enjoins good and forbids evil, is he an extremist? Such a person has done nothing other than fulfilling his religious duty. Now-a-days, not only some people, owing to some family and secular observations, are indifferent towards many religious obligations including amr bil-ma'ruf wa nahy anil munkar (enjoining good and forbidding evil) but they also label others as extremists and intolerant for fulfilling their duties. Therefore, it is necessary to know Shari'ah limits and if a person knows them, he will understand that crossing those limits amounts to extremism.
As for istikhara, we should tell you that istikhara is good by itself and it should be used in its appropriate place but when it comes to important decisions, one must first of all think and consult others. That is, it is necessary that one should think about the decision he is taking and then he should seek others' advice. He should consult experts of whose views and opinions he is confident. Then if he does not reach a decision and remains confused, he can turn to istikhara. Therefore, what some people are doing (for example some do istikhara whether to take their ailing relative to a doctor or not) is neither accepted by the religion nor is it reasonable. It is therefore necessary that you should carry out the first two stages. If you did not reach a conclusion, then you can do an istikhara.
 - The Prophet of Islam (pbuh) said:
ایّاکم و خضراء الدّمن قیل و ما خضراء الدّمن؟ قال المرأة الحسناء فی منبت سوء.
“O People! Beware of the green grass [growing] in a waste site.” Someone asked, “O Messenger of Allãh! And what is the green grass in a waste site?” He replied, “A beautiful woman in an evil environment.” Nahjul Fasahah (a collection of the Prophet's sayings), pg.355.
 - Abu Khalid Sajistani has narration from Imam Sadiq (a.s.) who said:
خمس خصال من فقد واحدة منهن لم یزل ناقص العیش، زائل العقل، مشغول القلب: فأولها صحة البدن، والثانیة الامن، والثالثة السعة فی الرزق، والرابعة الانیس الموافق -. قلت: وما الانیس الموافق؟ قال الزوجة الصالحة، والولد الصالح، والخلیط الصالح -. والخامسة وهی تجمع هذه الخصال: الدعة.
“Anyone who doesn’t have one of the following will always have an incomplete life; be unwise and always worried: health, security; abundance of daily bread; a sympathetic companion.” He was asked, “Who would constitute a sympathetic companion?” He replied, “A good wife; a good child or a good friend. And the last thing which includes all the others is comfort.” Shaykh Mufid, Al-Khisal,(English translation), Ansariyan Publications, pg.460.