I would recommend buying a USB modem (with connector for an external antenna) and a separate 3G-capable router for the following reasons:
- Routers with integrated 3G modems usually do not support so-called USSD commands which are often required to check or top up credit for prepaid SIM cards (something like *100#) and sometimes also to activate data options. A USB modem in contrast can be temporarily connected to a computer where you can submit such USSD commands. This spares you to remove the SIM card and put it in a phone each time you want to perform the afore-mentioned operations.
- You can use a USB extension cable (but not longer than 3m) between the router and the USB modem which often allows to use a shorter antenna cable as the modem can be placed in a less accessible place than the router has to be.
- Once attached to a computer you can perform manual site surveys with a USB modem.
- You can easily take the USB modem with you and use it with your computer on the go.
- In case of failure router and modem can be replace separately.
- Should LTE (aka 4G) become available at your place you can just replace the USB modem and keep your router.
- A separate router and USB modem is usually cheaper than a router with integrated modem.
That said my recommendation would be a TP-Link TL-MR3220 router and a Huawei USB modem that is
compatible to the MR3220 and features an antenna connector, such as the Huawei E173, E353 or E367.
Unfortunately Huawei, like most vendors, use a different antenna connector type, namely CRC9, but there are adaptors from SMA/RP-SMA to CRC9. It would be better to use a Yagi with shorter cable and a factory-made CRC9-connector instead of using an adaptor, such as the following:
15dBi UMTS Richtantenne YAGI 5 m Kabel CRC9 3G Huawei | eBay