u know pick and speed are opposite in a sense that if u want pick on ur 70 u will have to compromise on speed and vise versa ..
i would recomend u try out 13 tooth front sprocket .. do not change rear sprocket.. its the ratio of teh sprockets that matters . so changnig front is easy and front sprocket costs 50-80rs ..
the things works like this..
if u u have 14tooth front and 41 tooth rear sprockets the ratio is 41/14 or 2.92 what this figure means is that for one complete rotation of rear sprocket (wheel) the front sprocket will rotate 2.92 times...
if u want pick which is something like the ability to climb a steep slope then u increase the ratio which can be achieved by either increasing teh number of teeth on rear sprocket or reducing the teeth on front sprocket..
all in all i would suggest u to buy 13 and 15 tooth front sprocket and try each one for few days to know how it feels.. u will surely like one combo .. front sprockets cost 50-80 rs..