Mobile manufactures like Apple offer no option to extend the storage using memory cards. Same strategy is being followed by One Plus , Xiomi Mi5 and others. Choosing higher storage phones forces users to spend more bucks from the wallet. On the other hand most of the Android phones give users option to extend storage in the form of memory cards.
All memory cards with class X are not the same. Similar to the example 'All bikes of 125CC are not one and same' in terms of usability and performance. Going to local bazar for puchasing memory cards is not suggested as they sell only low speed memory cards.
Speed of a memory card is expressed in MBPS ( Mega Bytes Per Second ). Memory cards are categorized into Class 2, 4, 6, 8 and 10. Here Class 6 refers to 6MBps speed. To get a smooth experience in handling video recording of 720p and 1080p, you need atleast Class 6 ( sometimes class 4 is sufficient if bit rate is less) cards. You can see the class rating on memory card wrapped inside the C symbol. This speed is actually Write speed which will be very less comparable to Read speed. Above 10MBps(UHS-I), there are 30, 60 and 90 MBps cards which fall under UHS 3, V60 and V90 categories. Simply to put, write speed is usefull only when you record high definition videos or copying files to memory cards. V6 refers to 6MBps Video Recording Class.
32GB UHS-1 or Class 10 Samsung card is priced around Rs. 700 when the same capacity 32GB UHS-3 card is priced at 2350 in some popular e-commerce site. UHS-3 cards are suitable for 4K Videos and above. Unless your phone has the processing power and camera to record 1080p and 4K videos, do not go for costly memory cards as there will be no difference to feel.