From what I have heard about these things the only advise I can offer, look up the details of each one, how the are put together regards SN, from what I have heard about some & you will have seen the likes of HGVs ending up down lanes, streets that have become one-way, Streets are no-longer there, You can bye cheap but what normally happens when you bye cheap? you end up buying another, If you are intending to bye one & money is no problem, go & do your Internet homework & go onto Sat Nav forums.