One possibility is to get the train from the airport to centre town rail station(train station is signposted from arrivals, is a about a 3 minute walk with your luggage trolley). There are about 4 trains an hour I think.If you have already a bus/train pass then its valid already for this train, otherwise the ticket costs 3f and gives you one hour transport, which is more than enough for reaching town(~5-7 minutes), and taking the number 16 tram from in front of hotel Cornavin NEXT to train station(NOT the tram lines directly in front of the station), destination 'Moillesulaz'. Leaves every 10 minutes. It is the only tram to leave from here.
The journey should take about 9 minutes. You get off this tram at stop called ‘Terassière’. - the seventh stop I think. All stops are announced out loud on the trams (Terassière is the one just after one you can’t miss called ‘Rive’ - a major stop with long shelters and the car road going round it as a roundabout).
When you get off at Terassière, your hotel’s road is 2nd streets further, on the same side of the road as you are on.
You could take the train to the city station then get a taxi from there to hotel also as another possibility.
Of course if you have lots of luggage you may prefer the taxi full stop, as the hotel suggests. They will be familiar with the tarifs and 40 francs from the airport is probably a usual rate these days. I’d ask the driver for a receipt if you are in doubt.
I haven't taken one for a while myself, but only thing comes to mind regarding taxi is that there are major roadworks on one of the major roads down from airport to town for installation of trams. This might mean that the taxi may be slightly longer course until this is finished due to different route he/she would take, which may show up in a slight price rise.(???)
Enjoy your stay in Geneva!